SCIENCE  OF 
PLANT  LIFE 

Tra n s e a u 


BIOLOGY 

LIBRARY 

6 


SCIENCE    OF    PLANT    LIFE 

A  HIGH  SCHOOL  BOTANY 

TREATING  OF  THE  PLANT  AND  ITS 
RELATION  TO   THE   ENVIRONMENT 


NEW-WORLD  SCIENCE  SERIES 

Edited  by  John  W.  Ritchie 


SCIENCE  FOR  BEGINNERS 

By  Delos  Fall 
TREES,  STARS,  AND  BIRDS 

By  Edwin  Lincoln  Moseley 
COMMON  SCIENCE 

By  Carleton  W.  Washbume 
HUMAN  PHYSIOLOGY 

By  John  W.  Ritchie 
SANITATION  AND  PHYSIOLOGY 

By  John  W.  Ritchie 

LABORATORY  MANUAL  FOR  USE  WITH 
"HUMAN  PHYSIOLOGY" 

By  Carl  Hartman 

EXERCISE  AND  REVIEW  BOOK  IN  BIOLOGY 

By/.  G.  Blaisdell 
PERSONAL  HYGIENE  AND  HOME  NURSING 

By  Louisa  C.  Lippitt 
SCIENCE  OF  PLANT  LIFE 

By  Edgar  Nelson  Transeau 


ZOOLOGY 

By  T.  D.  A.  Cockerell 
EXPERIMENTAL  ORGANIC  CHEMISTRY 

By  Augustus  P.  West 


NEW-WORLD    SCIENCE    SERIES 

Edited  by   John   W.   Ritchie 

SCIENCE 
OF   PLANT   LIFE 

A  High  School  Botany 

Treating  of  the  Plant  and  Its  Relation 

to  the  Environment 

by 
Rdgar  Nelson  Trans  eau,  Ph.  £); 

Professor  of  Botany,   The  Ohio  State  University 


ILLUSTRATED 

with  engravings,  diagrams,  and  maps 
and  with  120  original  drawings  by 

Robert  J.  Sim 


Yonkers-on-Hudson,  New  York 

WORLD   BOOK    COMPANY 

1921 


WORLD    BOOK    COMPANY 

THE    HOUSE    OF    APPLIED    KNOWLEDGE      BIOLOGY 

' 

Established,  1905,  by  Caspar  W.  Hodgson 
YONKERS-ON-HUDSON,  NEW  YORK 
2126    PRAIRIE   AVENUE,   CHICAGO 


World  Book  Company  offers  Science  of  Plant 
Life  for  high  school  use.  The  book  em- 
bodies a  response  to  the  attractiveness  of 
plant  life,  a  steadfast  scientific  spirit  that 
yields  neither  to  sentiment  nor  to  utility, 
and  an  appreciation  of  the  fact  that  the  fun- 
damental reason  for  giving  botany  a  place  in 
our  general  scheme  of  education  is  that  it  is 
the  natural  scientific  background  for  the 
great  plant-producing  arts.  It  is  a  text  that 
accords  well  with  our  motto,  "  Books  that 
apply  the  world's  knowledge  to  the  world's 
needs  " 


MAIN  LIBRARY      —  •'.-.         TUtfEDEPT. 


NWSS:TSPL-4 


Copyright,  1919,  by  World  Book  Company 

Copyright  in  Great  Britain 

All  rights  reserved 


K  •. 


PREFACE 

THE  most  important  question  that  confronts  the  teacher  of 
elementary  botany  is  the  selection  of  the  subject  matter  for 
the  course.  Shall  the  work  consist  chiefly  of  the  naming  of 
plants  and  of  learning  the  meanings  of  terms  descriptive  of 
plants  and  of  plant  organs,  as  was  common  30  years  ago? 
Does  a  course  emphasizing  a  study  of  the  anatomy  of  plant 
organs  provide  the  best  introduction  to  the  subject  of  botany 
for  pupils  of  secondary  school  grade?  Have  those  courses 
been  satisfactory  in  which  the  work  centered  about  the  evolu- 
tionary development  of  the  plant  kingdom,  with  studies  of 
the  reproductive  organs  and  life  histories  of  types  representa- 
tive of  the  great  plant  groups,  or  shall  we  teach  physiology 
and  ecology?  Shall  we  select  as  material  for  study  wild 
plants,  often  obscure  and  unfamiliar  to  the  pupil,  because 
they  show  certain  structures  significant  in  determining  the 
relationships  of  plants,  or  shall  we  use  the  familiar  plants 
of  the  farm  and  garden,  on  which  man  depends  in  large  part 
for  his  livelihood,  to  exemplify  botanical  principles?  Is  it 
best  to  try  to  give  a  " practical"  turn  to  the  course  by  insert- 
ing chapters  from  other  subjects  like  agriculture,  forestry,  and 
plant  breeding ;  or  shall  the  course  be  kept  within  the  strict 
confines  of  botany  and  the  relation  of  botanical  facts  and 
principles  to  plant  production  be  shown  by  appropriate  refer- 
ences and  illustrations  ?  Upon  the  answers  given  to  these 
questions  the  content  of  the  course  to  be  offered  will  largely 
depend. 

The  author  is  one  of  those  who  think  that  our  work  in 
botany  should  serve  as  a  basis  for  agriculture,  horticulture, 
and  forestry,  just  as  physics  and  chemistry  form  the  natural 
background  of  our  manufacturing  and  industrial  life.  His  teach- 
ing experience  has  led  him  to  believe,  moreover,  that  a  very 

465784 


vi  Preface 

general  course  in  botany  that  presents  and  organizes  the  more 
important  facts  of  plant  physiology,  morphology,  ecology,  and 
economics  is  interesting  to  pupils  of  high  school  age  and  is 
valuable  to  any  one  who  later  engages  in  the  growing  of 
plants.  The  fundamental  aim  of  ihvs  text,  therefore,  is  to  give 
the  pupil  an  understanding  of  how  a  plant  lives  and  is  affected 
by  its  environment.  The  nutrition  of  the  plant  is  the  central 
theme.  Sufficient  anatomy  and  morphology  have  been  intro- 
duced to  make  possible  a  discussion  of  the  important  plant 
processes,  including  reproduction.  The  environment  of 
plants  and  their  adjustments  to  various  environmental  fac- 
tors have  been  discussed  because  a  knowledge  of  these  subjects 
is  essential  to  an  understanding  of  many  agricultural  practices. 
Attention  is  called  to  the  uses  that  are  made  of  plants  and 
plant  materials  and  to  the  applications  of  botanical  prin- 
ciples in  plant  production  in  order  that  the  economic  im- 
portance of  plants  and  of  a  knowledge  of  plant  life  may  be 
evident.  In  the  final  chapter  an  attempt  is  made  to  give 
the  pupil  an  understanding  of  what  is  meant  by  the  term 
" evolution"  —  to  present  a  definition  rather  than  a  discussion. 
The  most  valuable  part  of  any  course  in  botany  is  the  study 
of  plants  in  the  field  and  laboratory.  It  is  through  the  work 
with  the  plants  themselves  that  the  teacher  has  the  best 
opportunity  to  give  pupils  the  insight  into  plant  life  that  will 
justify  the  expenditure  of  their  time  and  energy  on  the  course. 
The  laboratory  work  has  been  fairly  well  standardized  in 
secondary  schools,  and  the  methods  of  handling  classes  are 
well  known.  This  is  not  true  of  field  work,  although  it  is 
no  more  difficult  than  laboratory  work  and  is  no  less  im- 
portant. Two  methods  of  conducting  field  observation  have 
been  eminently  successful.  One  is  to  take  the  class  into  the 


Preface  vii 

field  to  study  a  particular  topic,  —  not  to  make  random  ob- 
servations on  plants.  A  second  method  is  to  give  each  pupil 
a  carefully  prepared  outline  containing  questions  and  direc- 
tions by  which  he  can  make  a  trip  and  discover  for  himself 
the  answers  to  the  questions.  Both  these  methods  deserve  to 
be  used  far  more  than  they  are. 

This  book  has  been  written  to  supplement  laboratory  and 
field  work  with  plants,  not  to  take  the  place  of  such  work. 
Suggestions  for  laboratory  and  field  work  will  be  found  pre- 
ceding each  chapter,  but  these  suggestions  include  more  work 
than  can  be  accomplished  in  most  high  school  courses.  The 
teacher  should,  therefore,  select  those  exercises  that  are  best 
suited  to  the  needs  of  a  particular  class,  and  which  are  best 
adapted  to  the  laboratory  equipment  and  the  plant  material 
available. 


ACKNOWLEDGMENTS 

THE  author  wishes  to  thank  many  friends  and  students  who 
have  made  suggestions  regarding  the  book.  Especially  is  he 
under  obligation  to  the  following,  who  have  critically  read  the 
manuscript  or  the  proofs :  Dr.  Harris  M.  Benedict,  University  of 
Cincinnati;  Dr.  J.  M.  Lewis,  University  of  Texas;  Dr.  Burton 
E.  Livingston,  Johns  Hopkins  University ;  Dr.  S.  O.  Mast,  Johns 
Hopkins  University;  Dr.  Charles  A.  Shull,  University  of  Ken- 
tucky; Dr.  A.  B.  Stout,  New  York  Botanical  Garden;  Dr.  G. 
H.  Transeau,  Columbus,  Ohio;  to  his  colleagues  at  Ohio  State 
University,  especially  Dr.  Jay  B.  Park,  Dr.  H.  C.  Sampson, 
Dr.  A.  E.  Waller,  Professor  W.  G.  Stover,  and  Mr.  Paul  Sears ; 
to  Mr.  G.  W.  Salisbury,  Principal  of  Atchison  County  High 
School,  Emngham,  Kansas;  Mr.  J.  R.  Locke  and  Miss  Edith 
E.  Pettee,  Highland  Park  High  School,  Detroit,  Michigan ;  Miss 
Ella  M.  Bennett,  High  School,  Ann  Arbor,  Michigan;  Miss  Lucy 
M.  Phelon,  High  School  of  Commerce,  Springfield,  Massachu- 
setts ;  Miss  Ruth  Jackson,  High  School,  Wichita,  Kansas ;  and 
Miss  Maude  Flynn,  High  School,  Columbus,  Ohio. 


viii 


CONTENTS 


CHAPTER 


PAGE 


1.  PLANT  LIFE  IN  GENERAL      ...  i 

2.  LEAVES  AND  THEIR  STRUCTURES   .  .13 

3.  THE  MANUFACTURE  OF  FOOD        .        .        .  .        -25 

4.  LEAVES  IN  RELATION  TO  LIGHT  .      37 

5.  THE  WATER  RELATIONS  OF  LEAVES     .  .      48 

6.  LEAF  COLORATION  AND  THE  FALL  OF  LEAVES      .  .      63 

7.  DIGESTION,  TRANSFER,  AND  ACCUMULATION  OF  FOODS        .      74 

8.  THE  UTILIZATION  OF  FOODS          ....  .82 

9.  HERBS,  SHRUBS,  AND  TREES          .  .      94 

10.  STEMS  AND  THEIR  EXTERNAL  FEATURES      .  .    102 

11.  THE  STRUCTURES  AND  PROCESSES  OF  STEMS         .        .        .118 

12.  THE  ENVIRONMENT  OF  PLANTS     ...  .    139 

13.  ECOLOGICAL  GROUPS  OF  STEMS     .        .        .        .        .        .     154 

14.  THE  STRUCTURES  AND  PROCESSES  OF  ROOTS        .        .        .166 

15.  ROOTS  AND  THEIR  ENVIRONMENT 182 

1 6.  REPRODUCTION  IN  FLOWERING  PLANTS:     FLOWERS,  FRUITS, 

AND  SEEDS 197 

17.  REPRODUCTION  IN  RELATION  TO  AGRICULTURE     .        .        .217 

18.  THE  ALG^E 234 

19.  BACTERIA  AND  FUNGI 249 

20.  LIVERWORTS  AND  MOSSES      .    , 272 

21.  THE  FERNS  AND  THEIR  ALLIES 283 

22.  SEED  PLANTS:    GYMNOSPERMS       .        .  '     .        .        .        .     294 

23.  SEED  PLANTS:   ANGIOSPERMS 303 

24.  THE  EVOLUTION  OF  PLANTS 318 

INDEX .    331 


Flower  in  the  crannied  wall, 

I  pluck  you  out  of  the  crannies, 

I  hold  you  here,  root  and  all,  in  my  hand, 

Little  flower  —  but  if  I  could  understand 

What  you  are,  root  and  all,  and  all  in  all, 

I  should  know  what  God  and  man  is. 

TENNYSON 


SCIENCE    OF    PLANT    LIFE 

CHAPTER   ONE 

PLANT  LIFE  IN   GENERAL 
PLANTS    FROM    OUR    STANDPOINT 

PROBABLY  all  of  us  make  our  most  frequent  contact  with 
nature  through  plants,  and  no  part  of  our  environment  ap- 
peals to  us  more.  The  city  dweller  responds  to  the  attrac- 
tion of  plants  by  growing  a  few  flowers  in  a  window  box. 
The  suburban  resident  finds  great  pleasure  in  his  lawn  and 
trees.  The  farmer  enjoys  the  annual  miracle  of  transforming 
his  bare  fields  into  acres  of  productive  wheat  and  corn ;  and 
the  forester  delights  in  his  work  with  the  largest  and  most 
imposing  of  plants.  This  universal  challenge  of  plant  life 
is  reflected  in  our  literature;  the  stories  of  our  childhood 
days  and  the  novels  and  verse  of  our  maturer  years  are 
made  vivid  by  their  backgrounds  of  garden  and  meadow,  or 
of  forest  and  desert. 

The  importance  of  plants.  There  are  many  reasons  for 
studying  and  understanding  plants  besides  the  fact  that 
they  afford  us  pleasure. 

(1)  Plants  furnish  all  the  food  there  is  in  the  world.     Of  all 
living  beings,  green  plants  alone  are  able  to  organize  the 
simple  materials  found  in  the  air,  water,  and  soil  into  the 
complex  substances  which  all  plants  and  animals  must  have 
for  food.     The  part  of  our  own  food  which  is  not  derived 
from  plants  comes  from  animals  that  directly  or  indirectly 
feed  upon  plants. 

(2)  By  far  the  greater  part  of  all  the  fabrics  we  use  in  the 
making  of  clothing  is  woven  out  of  cotton,  linen,  and  other 


2  Science  of  Plant  Life 

plant  fibers ;   and  wool  and  silk  come  from  animals  that  feed 
on  plants. 

(3)  The  trees  supply  the  lumber  that  is  used  for  the  con- 
struction of  most  houses,  and  even  when  houses  are  built 
of  stone  or  brick,  wood  is  employed  in  finishing  the  interiors 
and  in  making  the  furniture.     Wood  is  used  also  in  the 
manufacture  of  paper,  and  in  almost  countless  other  ways. 

(4)  Most  houses  are  heated  in  winter  by  the  burning  of 
wood,  coal,  or  gas.     The  energy  used  in  the  driving  of  nearly 
all  machinery  is  derived  from  wood,  coal,  petroleum,  and 
natural  gas.     When  we  burn  wood,  we  release,  in  the  form 
of  heat,  the  great  store  of  energy  which  the  tree  obtained 
from  the  sunlight  during  its  lifetime.     When  we  burn  coal, 
petroleum,  or  natural  gas,  we  release  energy  which  plants 
accumulated  from  the  sunlight  of  millions  of  years  ago. 

(5)  Certain  small  plants  have  other  and  quite  different  re- 
lations to  human  beings,  and  their  activities  are  of  the  greatest 
consequence  to  man.     These  particular  plants,  the  bacteria, 
are  so  minute  that  they  can  be  seen  only  by  the  use  of  the 
microscope.      Some  of  them  take  nitrogen  from  the  air  and 
build  it  into  compounds  that  enrich  the  soil.     Others  render 
a  useful  service  by  breaking  down  the  dead  bodies  and  waste 
materials  of  plants  and  animals  and  converting  them  into 
substances  that  can  be  used  by  green  plants  in  the  making 
of  foods.     Still  other  kinds  of  bacteria  cause  many  of  the 
diseases  to  which  plants  and  animals  are  subject,  and  it  is 
necessary  for  us  to  learn  about  them  in  order  that  we  may 
avoid  or  destroy  them. 

We  see,  therefore,  that  plants  contribute  to  the  pleasure 
of  life ;  that,  directly  or  indirectly,  they  furnish  us  with  food 
and  provide  most  of  our  clothing  and  shelter ;  that  they  sup- 


Plant  Life  in  General 


trees  and  shrubs. 


ply  the  greater  part  of  the  energy  used  in  heating  and  lighting 
and  in  running  machinery ;  and  that  as  friends  or  foes,  cer- 
tain microscopic  plants  affect  the  health  and  well-being  of 
animals  and  men.  Since  plant  life  is  essential  to  our  very 
existence,  we  should  have  a  truly  vital  interest  in  the  science 
of  botany,  which  has  grown  out  of  the  accumulated  mass  of 
information  about  plants. 

Botany  as  a  science.  A  science  is  a  body  of  classified  and 
systematized  facts.  The  lore  of  the  gardener,  the  forester, 
and  the  casual  student  of  plants  may  have  value,  but  it  be- 
comes a  science  only  when  it  has  been  organized  into  a  system 
whose  principles  have  been  tested  and  verified  by  experience, 
observation,  or  experiment.  Botany  is  the  science  that 
treats  of  the  structures,  life  histories,  physiological  processes, 
distribution,  and  classification  of  plants.  It  is  one  of  the  two 
divisions  of  biology;  zoology,  the  science  of  animal  life,  is 


4  Science  of  Plant  Life 

the  other.  Because  so  many  of  the  fundamental  processes 
and  structures  in  plants  and  animals  are  similar,  it  is  pos- 
sible to  include  all  living  beings  in  the  single  science  of  biology. 
Yet  because  plants  and  animals  differ  in  many  important 
ways,  botany  and  zoology  may  well  be  considered  as  distinct 
sciences. 

How  a  knowledge  of  botany  helps  us.  A  knowledge  of 
botany  contributes  directly  to  our  enjoyment  of  life,  because 
the  more  we  know  about  plants,  the  more  interest  and  mean- 
ing we  find  in  every  bit  of  vegetation.  Botany  helps  us  to 
understand  the  animal  world,  also,  for  plants  and  animals 
are  so  nearly  related  that  much  of  what  we  learn  in  botany 
applies,  with  slight  modifications,  to  animals  as  well  as  to 
plants. 

Botany  has  great  practical  value  also ;  it  furnishes  the 
scientific  basis  for  many  of  the  most  important  of  human  oc- 
cupations. For  example,  agriculture  and  horticulture  are 
arts  dealing  with  the  methods  of  field  and  garden  crop  pro- 
duction; they  tell  how  and  when  a  crop  shall  be  planted, 
cared  for,  and  harvested ;  but  it  is  botany  that  furnishes  the 
scientific  knowledge  on  which  these  arts  are  based  and  explains 
the  principles  underlying  the  practices  of  the  gardener  and  the 
farmer.  Botany  does  not  tell  us  of  the  methods  to  be  used 
by  the  forester  in  developing  timber;  but  no  forester  can 
practice  intelligently  or  invent  new  and  better  methods  with- 
out understanding  the  principles  of  botany.  How  to  make 
a  city  or  farm  dwelling  sanitary  is  an  engineering  problem ; 
but  the  engineer  must  be  thoroughly  familiar  with  at  least 
the  part  of  botany  which  deals  with  the  bacteria,  if  he  is  to 
be  intelligent  in  his  work.  Botany,  therefore,  is  of  great 
practical  importance  in  our  daily  life ;  without  a  knowledge 


Plant  Life  in  General 


FIG.  2.    A  perennially  beautiful  garden. 

of  it  many  of  our  most  important  arts  can  be  practiced  only 
in  a  cumbersome  way. 

Nothing  has  as  yet  been  said  about  our  natural  curiosity 
as  an  incentive  to  the  study  of  plants.  Yet  the  desire  to 
know  and  understand  is  so  strongly  developed  in  all  human 
beings  that  it  has  probably  had  more  to  do  with  the  growth 
of  the  sciences  than  all  other  influences  combined.  We 
naturally  want  to  understand  why  things  are  as  we  find  them ; 
we  are  ever  seeking  explanations  of  peculiar  objects  and  un- 
usual happenings  that  come  to  our  attention.  The  study  of 
botany  will  help  to  satisfy  our  wholesome  curiosity  about 
plants,  and  will  direct  our  inquiries  into  profitable  channels. 

Newspapers  and  magazines  often  publish  accounts  of 
strange  plants  or  of  unusual  plant  habits.  We  read  that  wheat 
found  in  the  tombs  of  ancient  Egypt,  where  it  had  been  buried 
for  many  centuries,  was  still  alive.  It  is  reported  that  a  tree 


Science  of  Plant  Life 


FIG.  3.    The  vegetation  more  than  the  house  gives  character  to  this 
suburban  home. 

that  captures  men  and  large  animals  and  feeds  on  them  has 
been  found  in  central  Africa.  We  are  informed  that  a  spine- 
less cactus  has  been  produced  from  a  spiny  one  "  by  treating 
it  kindly."  Such  published  accounts  and  the  descriptions  and 
explanations  that  accompany  them  are  purposely  wrapped 
in  mystery,  and  tend  to  give  wrong  impressions  of  plant 
life.  Even  a  little  knowledge  of  botany  will  keep  one  from 
being  misled  by  such  statements.  In  common  with  all  the 
other  sciences,  botany  enables  us  to  recognize  the  truth,  and  so 
helps  us  to  avoid  errors  due  to  mistaken  observation,  im- 
perfect knowledge,  or  willful  attempts  to  deceive. 

PLANTS   FROM   THEIR   OWN   STANDPOINT 

Thus  far  plants  have  been  discussed  in  relation  to  man ; 
they  have  been  considered  as  objects  of  interest,  and  as  a 


Plant  Life  in  General  7 

part  of  man's  environment  that  may  promote  or  interfere 
with  his  welfare.  Of  course  plants  do  not  grow,  or  flower, 
or  fruit  for  the  sake  of  the  animals  and  man.  The  processes 
of  plants  are  carried  on  and  their  structures  developed  solely 
to  meet  their  own  needs.  The  goal  of  plant  life  is  the  de- 
velopment of  the  individual  and  the  production  of  young 
for  the  continuance  of  the  species.  A  plant  is  successful  in 
nature,  therefore,  (i)  when  it  secures  nourishment  for  its 
complete  development,  and  (2)  when  it  produces  offspring 
and  thus  insures  the  reproduction  of  its  kind. 

Plants  as  living  things.  It  is  important  for  the  beginner  in 
the  study  of  botany  to  realize  that  plants  are  living  things. 
Because  animals  walk,  or  fly,  or  swim  about,  we  are  accus- 
tomed to  think  of  movement  as  the  necessary  evidence  of 
life.  To  one  who  has  given  no  thought  to  the  subject,  a  tree 


FIG.  4.     Rope-making  scene  in  a  Philippine  village.     Out  of  plant  materials  the  Fili- 
pinos make  houses,  mats,  cloth,  boats,  and  a  great  variety  of  household  utensils. 


8  Science  of  Plant  Life 

may  seem  more  akin  to  the  stones  among  which  it  is  rooted 
than  to  the  animals  that  live  about  it.  But  when  we  study 
living  beings,  we  find  that  there  are  activities  other  than 
movement,  such  as  respiration  and  growth,  that  are  regularly 
associated  with  life.  As  we  shall  see  later,  these  processes 
take  place  in  plants  the  same  as  in  animals  and  are  evidence 
that  plants  are  as  truly  alive  as  are  animals. 

A  plant  definitely  related  to  its  environment.  The  roots 
of  the  ordinary  green  plant  penetrate  the  soil  in  all  directions 
from  the  base  of  the  plant,  and  enable  it  to  take  up  water 
and  mineral  substances.  The  stem  commonly  grows  upward 
and  supports  the  leaves.  Thus  the  leaves  are  displayed  to 
sunlight  and  are  in  contact  with  the  oxygen  needed  for  res- 
piration and  the  carbon  dioxid  required  for  the  making  of 
food.  Each  part  of  the  plant  is  related  to  its  environment  in 
such  a  way  that  its  natural  processes  and  the  life  of  the  plant 
as  a  whole  may  be  carried  on. 

Mutual  dependence  of  the  parts  of  a  plant.  The  roots, 
stems,  and  leaves,  together,  make  up  the  plant's  machinery 
of  nutrition,  and  upon  the  efficiency  with  which  the  work  of 
each  part  is  done  depends  the  successful  nourishment  of  the 
plant.  The  chemical  nature  of  the  soil,  the  amount  of  water 
it  contains,  and  its  other  characteristics,  may  facilitate  or 
hinder  the  work  of  the  roots.  This  may  in  turn  aid  or  inter- 
fere with  the  work  of  the  leaves,  and  as  a  result  the  whole 
plant  may  flourish  or  be  dwarfed.  Likewise,  the  leaves  may 
be  exposed  to  favorable  or  unfavorable  conditions  of  light 
and  moisture,  and  their  work  may  be  accelerated  or 'retarded. 
This  in  turn  affects  the  stem  and  the  roots,  and  the  whole 
plant  shows  its  abundant  nutrition  or  its  lack  of  food. 

Under  favorable  conditions  of  light  and  moisture  corn  may 


Plant  Life  in  General 


FIG.  5.     Plants  furnish  the  primary  food  supply  of  the  world.     The  photograph 
shows  a  rice-harvesting  scene  in  the  Philippine  Islands. 

be  grown  in  sand ;  but  because  sand  does  not  provide  the 
plant  with  the  necessary  mineral  substances,  the  corn  will 
not  be  of  normal  growth  and  will  fail  to  produce  good  seed. 
Under  the  same  conditions  of  light  and  moisture,  a  rich  gar- 
den soil  would  produce  a  normal  plant.  Or,  if  during  the 
winter  months  corn  is  grown  in  the  richest  of  soil  in  a  green- 
house, it  attains  only  a  small  size  and  may  fail  to  produce  good 
seeds,  because  in  this  case  the  amount  of  light  is  not  sufficient. 
Hence,  when  we  discuss  the  relations  of  any  particular  part 
of  a  plant  to  the  energy-supplying  and  nutritive  processes, 
we  must  ever  keep  in  mind  the  interrelation  and  mutual  de- 
pendence of  all  parts  of  the  plant.  As  no  part  of  the 
human  body  lives  an  independent  life  but  is  dependent  for 


io  Science  of  Plant  Life 

its  welfare  on  the  activities  of  the  other  parts,  so  the  life 
of  each  part  of  a  plant  is  bound  up  with  the  life  of  the  plant 
as  a  whole. 

Reproduction  an  essential  process  in  plant  life.  Plants, 
to  be  successful,  not  only  must  maintain  themselves  but  they 
must  reproduce  themselves.  Some  of  them  do  this  by  fur- 
ther development  of  a  part  of  the  parent  body,  as  the  tuber 
of  a  potato  or  the  runner  of  a  strawberry  plant.  In  many 
plants,  however,  reproduction  takes  place  only  through  the 
production  of  flowers,  fruits,  and  seeds.  These  structures  in 
one  way  or  another  are  concerned  with  the  production  of  a 
small,  undeveloped  plant  within  the  seed,  and  it  is  upon  the 
further  growth  of  this  young  plant  that  the  production  of 
another  generation  of  that  particular  kind  of  plant  depends. 
A  sunflower  may  develop  a  tall  stem  and  a  large  leaf  area, 
but  unless  it  flowers  and  produces  good  seed  no  young  plants 
can  be  grown  from  it.  If  it  were  the  only  sunflower  in  ex- 
istence, there  could  be  no  more  sunflowers  after  its  death. 
The  process  of  reproduction  must,  therefore,  be  considered 
as  an  essential  one  in  plants;  without  it  plant  life  would 
soon  disappear  from  the  earth. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Two 

1.  Review:    physical  and  chemical   changes;    molecules  and 
atoms ;   elements  and  compounds ;   solids,  liquids,  and  gases.     If 
the  pupils  have  had  no  introductory  science,  a  few  simple  demon- 
strations should  be  given  to  make  these  topics  clear. 

2.  Study  a  complete  plant,  noting  the  roots,  stems,  leaves, 
flowers,  fruits,  and  seeds.     Many  of  the  common  weeds  show  all 
these  parts  clearly.     Examine  the  plant,  not  primarily  to  learn  the 
names  of  the  parts,  but  in  order  to  get  a  conception  of  it  as  a  living 
thing  that  is  very  different  from  the  air  and  the  soil  in  which  it 
grows.     Find  out  what  the  pupils  already  know  concerning  the 
plant's  requirements,  its  manner  of  growth,  and  the  functions  of 
its  several  parts. 

3.  Study  a  number  of  leaves  from  different  plants,  and  make 
drawings  to  illustrate  a  variety  of  leaf  forms,  simple  and  compound 
leaves,  and  the  parallel  and  net  arrangements  of  the  veins.     Note 
the  thickness,  texture,  color,  and  surface  in  each  of  the  different 
specimens.     Note  also  that  the  veins  reach  every  part  of  the 
leaf  and  connect  through  the  petiole  with  the  interior  of  the  plant 
stem. 

4.  Study  a  skeletonized  leaf  to  make  clear  the  arrangement  of 
veins  and  their  intimate  relation  to  the  mesophyll  cells.     A  nastur- 
tium leaf  that  has  been  decolorized  in  alcohol  may  be  studied  under 
the  microscope  to  show  the  small  veins  and  the  fine  ramifications 
of  the  vessels. 

5.  Dissect  a  leaf  from  an  Easter  lily,  live-for-ever,  or  Wandering 
Jew.     Study  especially  the  relative  positions  of  the  epidermis, 
mesophyll,  and  veins. 

6.  Study  with   a   microscope   cells   from   the   epidermis,   dis- 
tinguishing the  cell  wall,  protoplasm,  nucleus,   cytoplasm,  and 
vacuole.     The  epidermis  from  an  onion  scale  is  good  material 
for  the  study  of  the  cell  parts. 

7.  Draw  some  cells  from  a  leaf  of  moss,  such  as  Mnium,  after 
it  has  been  in  good  light  for  several  hours.     Note  cell  walls  and 
chloroplasts.     Staining  with  a  weak  iodin  solution  will  show  the 
starch  grains  within  the  chloroplasts. 


12  Science  of  Plant  Life 

8.  By  experiment  determine  that  light  is  necessary  for  the 
development  and  maintenance  of  the  green  color  in  leaves. 

9.  Study  a  freshly  cut  cross-section  of  a  leaf,  mounted  in  water, 
to  show  the  detailed  structure  and  the  intimate  connection  between 
the  epidermis,  mesophyll,  and  veins.     Prepared  sections  will  show 
details  of  the  stomata  and  intercellular  spaces. 

10.   Take  a  field  trip  to  study  the  variety  of  leaf  forms. 


CHAPTER  TWO 


LEAVES  AND  THEIR  STRUCTURES 

THE  leaves  of  plants  are  their  most  conspicuous  part.  The 
summer  landscape  owes  its  color  to  them ;  and  even  when  we 
look  at  a  near-by  plant,  the 
leaves  attract  most  of  our 
attention  and  the  plant  stem, 
like  the  staff  of  a  flag,  is  likely 
to  be  overlooked.  The  promi- 
nence of  leaves  is  not  the  result 
of  chance,  for  leaves  manufac- 
ture food  and  sunlight  is  neces- 
sary for  this  process.  In  this 
chapter  we  shall  study  the 
structure  of  a  leaf,  and  in  sub- 
sequent chapters  we  shall  dis- 
cuss the  work  of  the  leaves  and 
the  processes  that  take  place 
within  these  important  organs 
of  the  plant. 

The  parts  of  a  leaf.  If  we 
examine  a  leaf  closely,  we  see 
that  it  consists  of  a  broad,  thin  blade,  marked  into  small 
divisions  by  veins.  The  vein  near  the  middle  of  the  blade  is 
commonly  larger  than  the  others  and  is  called  the  midrib. 
In  some  forms  of  leaves  there  are  several  prominent  veins, 
which  we  may  call  the  principal  veins.  In  general,  the  small- 
est veins  form  a  network  uniting  with  the  larger  ones,  and 
these  in  turn  connect  with  the  midrib  or  with  the  principal 
veins.  These  large  veins  are  smallest  at  the  apex  or  outer 
end  of  the  leaf,  and  gradually  become  larger  toward  the  base 

13 


Stipul 


FIG.  6.    Leaf  of  tulip  tree,  showing 
parts  of  a  complete  leaf. 


Science  of  Plant  Life 


Margin 


of  the  blade.  They  continue  down  through  the  petiole  or 
leafstalk  into  the  interior  of  the  stem.  At  the  base  of  the 
Apex  petiole  there  is  in  many  leaves 
a  pair  of  small  appendages,  the 
stipules.  These  are  usually  un- 
important structures,  but  oc- 
casionally they  are  large  and 
bladelike,  and  supplement  the 
blade,  or  even  take  its  place,  in 
food  manufacture.  The  primary 
divisions  of  the  leaf  are  the  blade, 
the  petiole,  and  the  stipules. 

The  leaf  made  up  of  tissues. 
The  soft  green  tissue  essential 
to  food  production  is  found 
chiefly  in  the  blade  of  the  leaf. 
This  may  be  shown  by  dissect- 
ing a  fleshy  leaf  like  that  of  the  common  houseleek  or  the 
live-for-ever.  Cutting  across  the  blade  of  such  a  leaf,  we 


Lateral  vein 


FIG.  7.    Leaf  of  vinca,  showing  the 
parts  of  the  blade. 


FIG.  8.     Leaves  showing  variety  of  form  and  venation:  A,  orange;  B,  peach; 
C,  bamboo ;  D,  nasturtium ;  E,  poplar ;  F,  lantana ;   G,  begonia. 


Leaves  and  Their  Structures  15 

find  that  there  is  a  skin  covering  it  above  and  below.     The 
skin  is  readily  stripped  off,  leaving  the  interior  of  the  leaf 


FIG.  9.     Divided  and  compound  leaves:  A,  buckeye;  B,  oxalis;  C,  avens; 
D,  celandine ;  E,  cliff  fern ;  F,  dandelion. 

as  a  green,  granular  mass  of  cells  with  veins  running  through 
it  in  all  directions.  The  skin  is  called  the  epidermis,  or 
epidermal  tissue  (Greek:  epi,  upon,  and  derma,  skin).  The 
green  part  is  the  mesophyll  tissue  (Greek :  meso,  middle,  and 
phyll,  leaf).  The  veins  consist  of  three  tissues,  the  water- 
conducting,  food-conducting,  and  mechanical  tissues.  The 
blade  therefore  commonly  contains  five  tissues  :  the  epidermis 
and  mesophyll,  and  the  three  tissues  of  the  veins.. 

Cells.  When  any  one  of  the  tissues  of  a  leaf  or  other  living 
part  of  a  plant  is  magnified  under  a  microscope,  it  is  seen  to 
be  composed  of  small  parts  built  together  in  much  the  same 
way  as  the  little  chambers  in  a  honeycomb.  These  small 
parts  are  the  plant  cells  (Fig.  1 1) .  Each  cell  consists  of  a  small 
mass  of  jellylike  living  matter,  the  protoplasm,  which  is  inclosed 
by  a  firm,  transparent  wall.  The  protoplasm  is  divided  into 
a  denser  round  or  oval  body,  the  nucleus,  and  a  more  liquid 
portion,  the  cytoplasm.  The  nucleus  is  of  great  importance ; 


i6 


Science  of  Plant  Life 


the  cell  dies  when  it  is  removed,  and  it  is  thought  to  control 
many  of  the  activities  that  go  on  within  the  cell.     The  cells 


FIG.  10.     Leaves  with  prominent  stipules :  pea,  black  willow,  red  clover, 
Japanese  quince,  rose. 

are  the  structural  units  of  plants.  Figure  1 2  shows  how  a  leaf 
is  built  of  cells  of  different  sizes  and  shapes. 

The  cytoplasm  makes  up  the  bulk  of  the  living  matter  of 
a  cell,  but  in  mature  plant  cells  most  of  the  space  inclosed  by 
the  cell  wall  is  occupied  by  one  or  more  vacuoles  or  cavities 
containing  the  cell  sap.  This  is  water  with  sugars,  mineral 
salts,  acids,  and  other  substances  dissolved  in  it.  Lying 
within  the  cytoplasm  are  structures  called  plastids,  small 
bodies  that  contain  food  substances  and  coloring  matters. 

The  cell  wall.  The  wall  which  surrounds  the  cell  is  com- 
posed of  a  transparent  material  called  cellulose.  Its  impor- 
tance lies  in  the  fact  that  it  gives  firmness  to  the  cell.  It 


Leaves  and  Their  Structures 


supports  the  soft  cytoplasm  as  the  wax  of  the  honeycomb  sup- 
ports the  honey  within,  and  it  helps  to  give  stiffness  to  all 
parts  of  the  plant.  You  have 
seen  pure  cellulose  in  the  form 
of  cotton.  Filter  paper  and 
most  book  papers  are  made  of 
cellulose  fibers  derived  from 
wood.  Water  passes  freely 
through  the  cellulose  walls  of 
plant  cells,  as  do  most  substances 
that  are  dissolved  in  the  water. 
Animals,  as  well  as  plants,  are 
composed  of  cells ;  but  the 
animal  cell,  instead  of^  having  a 
stiff  cellulose  wall  like  a  plant 


cell,  has  a  soft  wall,  or,  as  in  the    FlG'  '.'•    ^  f'    A 

moss  leaf;    B  is  from  a  squash- vine 
Case     Of     nerve     Cells     and     white     hair;    C  is  a  starch-filled  cell  from  a 

blood  corpuscles,  it  may  lack  a    P°tato  tuber'  and  D  is  a  cel1  from 

the  palisade  layer  of  a  leaf.     E  shows 

wall  entirely.     Consequently,  the    a  ceu  in  cross  section, 
tissues   of    animals    (except   the 

skeletal  tissues)  are  usually  softer  and  more  pliable  than 
plant  tissues.  This  makes  it  easy  for  an  animal  to  bend  and 
to  move  about.  The  difference  in  cell  walls  and  in  the 
pliability  of  tissues  is  so  general  throughout  the  plant  and 
animal  kingdoms,  that  it  is  one  of  the  important  distinc- 
tions between  plants  and  animals. 

The  epidermis  and  the  stomata.  The  cells  of  the  epidermis 
are  flat,  irregularly  shaped,  closely  united,  and,  for  the  most 
part,  colorless.  The  cell  walls  on  the  side  of  the  epidermis 
which  is  exposed  to  the  air  become  thickened  with  a  waxlike 
material  called  cutin,  which  forms  a  layer  over  the  surface  of 


i8 


Science  of  Plant  Life 


the  leaf.     This  layer  is  called  the  cuticle.     It  is  useful  to  the 
plant  because  water  does  not  pass  through  it  readily,  and  it 


epide 


4J 


M 


b 


Vfeter-coiF 
ducting  tissu< 

Food- con- 
ducting tissue 

Bundle  sheatW     Guard  cell 


; 


Lower 
epidermis 


Stoma^7*><fF     -f 
FIG.  12.     Model  of  a  small  piece  of  vinca  leaf,  showing  cells  and  tissues. 

protects  the  plant  from  water  loss.  It  may  be  compared  to 
the  enamel  covering  of  oilcloth  and  acts  in  much  the  same 
way.  The  cuticle  is  useful  to  the  plant  also  because  it  serves 
as  a  first  line  of  defense  against  disease  germs.  The  impor- 
tance of  the  epidermis  as  a  protective  covering  for  the  delicate 
inner  tissues  of  the  plant  may  be  judged  from  the  drying  and 
decay  that  follow  the  breaking  of  the  thin  epidermal  coat  of 
an  apple  or  a  pear. 

Scattered  among  the  colorless  cells  of  the  epidermis  are 
pairs  of  small,  crescent-shaped  green  cells,  the  guard  cells. 
Each  pair  of  these  surrounds  a  small  opening  or  pore,  the 


Leaves  and  Their  Structures 


stoma   (Greek:    stoma,  mouth;    plural,   stomata),1    which  is 
opened  or  closed  by  the  expansion  or  contraction  of  the  guard 


FIGS.  13  and  14.  Upper  epidermis  of  "Wandering  Jew"  (Zebrina)  leaf,  on  the  left,  and 
lower  epidermis,  on  the  right.  St  is  a  stoma,  G  a  guard  cell,  and  Sc  a  subsidiary  cell. 
The  stomata  are  found  only  on  the  lower  surface  of  the  leaf. 

cells.  The  stomata  are  very  important,  for  they  connect  the 
air  spaces  among  the  cells  inside  the  leaf  with  the  external 
atmosphere.  When  open,  they  allow  the  exchange  of  water 
vapor  and  other  gases  through  the  epidermis;  and  when 
closed,  they  complete  the  barrier  to  gas  movements  in  either 
direction.  In  many  plants  stomata  occur  only  on  the  lower 
surfaces  of  the  leaves ;  but  in  some  plants  they  are  found  on 
both  the  upper  and  lower  leaf  surfaces. 

The  mesophyll.  The  mesophyll  tissue  is  composed  of  the 
soft,  thin- walled  cells  that  lie  among  the  veins  in  the  in- 
terior of  the  leaf.  In  most  leaves  there  is  beneath  the  upper 
epidermis  one  or  more  palisade  layers,  which  are  composed  of 

1  Stomata  are  so  small  that  2500  of  them  have  an  area  about  equivalent  to 
that  of  an  ordinary  pin  hole.  They  are  so  numerous,  however,  that  they 
occupy  about  ^  of  the  area  of  the  average  leaf.  On  a  square  centimeter 
of  the  lower  surface  of  a  sunflower  leaf  there  are  about  150,000  of  them. 


20  Science  of  Plant  Life 

elongated  cells  standing  close  together,  as  is  shown  in  Figure 
12.  The  remainder  of  the  mesophyll  tissue  is  made  up  of 
ovoid  or  irregularly  shaped  cells  joined  quite  loosely,  so  that 
air  spaces  are  left  between  them.  In  fact,  a  much  larger 
part  of  the  surfaces  of  these  cells  is  in  contact  with  air  spaces 
than  with  other  cells.  The  air  spaces  within  the  leaf  are  con- 
tinuous, and  through  them  the  oxygen  and  carbon  dioxid  of 
the  atmosphere  can  reach  every  cell  in  the  leaf.  We  shall 
see  later  that  the  differences  in  the  epidermal  and  mesophyll 
cells,  and  in  the  way  they  are  arranged,  are  definitely  related 
to  the  different  processes  carried  on  by  each  of  them. 

The  veins.  The  veins  in  a  leaf  branch  again  and  again, 
forming  a  fine  meshwork  through  all  its  parts.  Each  vein  is 
composed  of  a  bundle  of  water-conducting  and  food-con- 
ducting tissues  surrounded  by  a  bundle  sheath.  The  water- 
conducting  tissues  are  located  in  the  upper  side  of  the  vein. 
These  tissues  are  made  up  of  long,  cylindrical  cells  placed 
end  to  end.  Usually  the  inner  walls  of  these  cells  have 
spiral  thickenings,  and  sometimes  the  end  walls  of  the  cells 
are  absorbed,  leaving  continuous  tubes  or  vessels  several 
cells  in  length.  After  the  growth  of  the  cells  is  completed,  the 
living  protoplasm  within  them  dies,  and  the  dead  cases  of  the 
cells,  with  their  porous  walls,  lie  within  the  leaf  like  bundles  of 
very  fine  pipes.  Through  these  vessels,  the  water  and  mineral 
salts  that  are  absorbed  by  the  roots  pass  into  the  leaf  to 
supply  its  living  cells.  The  supplies  of  water  and  mineral 
salts  pass  out  through  the  walls  of  the  water-conducting  ves- 
sels into  the  cells  that  adjoin  them,  and  then  from  these  they 
pass  to  the  other  cells  of  the  leaf. 

The  food-conducting  tissues,  or  vessels,  lie  below  the  water- 
conducting  vessels  within  the  leaf  veins.  They  provide  an 


Leaves  and  Their  Structures 


21 


elaborate  system  of  channels  by  which   the  surplus  foods 
manufactured   in   the   leaf    are  distributed   throughout  the 


FIGS.  15  and  16.     Upper  and  lower  epidermis  of  vinca  leaf. 

plant.  The  foods  pass  from  the  mesophyll  cells  into  this 
food-conducting  tissue,  and  then  down  through  the  petiole 
of  the  leaf  to  the  living  cells  of  the  stem  and  roots. 

In  the  smaller  veins  the  bundle  sheath  is  a  layer  of  meso- 
phyll cells.  In  the  larger  veins  it  contains  one  or  more  layers 
of  thick-walled  cells,  which  act  as  a  mechanical  or  supporting 
tissue.  The  mechanical  tissue  is  rigid  and  gives  stiffness  to 
the  leaf. 

Cells,  tissues,  and  organs.  We  see,  then,  that  the  actual 
work  of  the  plant  is  done  in  its  cells,  of  which  there  are  many 
millions,  and  it  is  the  sum  of  the  life  and  work  of  all  these 
cells  that  makes  up  the  life  and  work  of  the  plant  as  a  whole. 
All  cells  carry  on  certain  fundamental  life  processes  like 
respiration  and  the  assimilation  of  food,  but  most  cells  are 
especially  adapted  to  some  particular  work  that  is  carried  on 
for  the  benefit  of  the  plant  as  a  whole.  Cells  that  have  the 
same  special  function  are  similar  in  structure  and  are  generally 


22  Science  of  Plant  Life 

grouped  together.  Such  groups  of  cells  with  like  functions 
are  called  tissues.  The  epidermis  of  a  leaf,  for  example,  is  a 
tissue  covering  the  mesophyll  and  veins. 

In  order  to  do  its  work  a  tissue  needs  a  source  of  supplies 
and  a  means  of  disposing  of  its  products.  Hence  the  group- 
ing of  the  tissues  may  be  mutually  advantageous.  When 
several  kinds  of  tissues  are  arranged  together  so  that  by  their 
cooperation  they  can  carry  on  some  general  function  of  the 
plant,  they  form  an  organ.  The  leaf,  for  example,  is  an  organ 
especially  concerned  with  the  manufacture  of  food.  It  is 
made  up,  as  we  have  seen,  of  five  different  tissues,  each  com- 
posed of  thousands  of  cells. 

The  chloroplasts.  Of  the  several  structures  found  within 
the  mesophyll  cells,  the  most  important  in  the  primary  process 
of  food  manufacture  are  the  chloroplasts.  These  are  round 
or  lens-shaped  bodies  which  contain  a  green  coloring  matter 
called  chlorophyll.  They  are  composed  of  living  material 
and  belong  to  a  group  of  structures  called  plastids,  that  are 
found  in  the  cytoplasm  of  all  plant  cells.  Cells  may  contain 
many  or  only  a  few  chloroplasts,  and  these  may  be  located 
deep  within  the  leaf  or  near  its  surface  (Fig.  17).  Since 
the  chloroplasts  are  the  special  apparatus  for  the  manu- 
facture of  food,  the  amount  of  food  produced  by  a  plant 
under  any  given  conditions  is  roughly  proportional  to  their 
number. 

The  chlorophyll.  Chlorophyll  is  held  in  the  chloroplasts 
in  much  the  same  way  that  water  is  held  in  a  sponge.  It 
stains  the  chloroplasts  green,  and  it  may  be  removed  from 
them  by  putting  the  leaf  in  alcohol,  in  which  the  chlorophyll 
is  soluble.  After  the  chlorophyll  is  dissolved,  the  chloroplasts 
remain  in  the  cell,  but  they  are  colorless  and  the  leaf  is  white 


Leaves  and  Their  Structures 


or  yellowish  instead  of  green. 
Light  is  usually  necessary  to 
the  development  of  chlorophyll. 
The  white  sprouts  on  potatoes 
in  a  dark  cellar,  the  blanching 
of  celery  when  the  lower  part  of 
the  leaves  is  covered,  and  the 
whitening  of  grass  under  a  board, 
are  familiar  evidences  of  this 
fact.  In  the  inner  tissues  of 
plants  and  in  the  underground 
parts,  the  plastids  are  usually 

Colorless  ;     but     in    many    plants       FIG.  17.     Part  of  a  moss  leaf  that  is 

these  parts  become  green  if  they       comP°sed  of  a  sing*e  layer  of  cells- 
are  exposed  to  the  light.     This  is  why  potatoes  that  grow  at 
the  surface  of  the  soil  are  likely  to  be  green. 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Three 

Answer  the  following  questions  by  means  of  experiments : 

1.  Do  leaves  grow  toward  the  light  ? 

2.  How  is  the  blade  moved  into  its  position  with  reference  to 
light? 

3.  Is  light  necessary  for  starch  manufacture? 

4.  Is  chlorophyll  necessary  for  starch  manufacture? 

5.  Does  the  manufactured  starch  remain  in  the  leaves  ? 


24 


CHAPTER  THREE 

THE   MANUFACTURE   OF  FOOD 

You  will  probably  remember  from  your  study  of  physiology 
that  all  the  foods  used  by  animals  belong  to  three  classes 
of  chemical  substances :  carbohydrates,  fats,  and  proteins. 
These  same  classes  of  substances  constitute  the  food  of  plants. 
A  grain  of  corn  contains  a  supply  of  Starch,  oil,  and  pro- 
tein for  the  young  plant,  and  these  same  foods  that  are  used 
by  animals  are  accumulated  in  many  plants.  The  difference 
in  the  nutrition  of  plants  and  animals  lies,  then,  not  in  any 
differences  in  the  foods  used,  but  in  the  way  their  foods  are 
secured.  In  this  chapter  the  manner  in  which  plants  obtain 
their  foods  will  be  discussed. 

Plants  the  source  of  all  food.  Mineral  soils  and  the  air 
do  not  contain  any  of  the  substances  that  we  class  as  foods. 
Yet  green  plants  may  grow  luxuriantly  on  mineral  soils.  It 
follows,  therefore,  that  green  plants  are  able  to  manufacture 
their  own  foods.  They  can  synthesize,  or  build  together, 
simple  substances  that  they  obtain  from  the  soil  and  air 
into  the  complex  foods  that  they  require.  Animals  lack 
this  power.  They  must  have  foods  that  have  already  been 
built  up,  rather  than  the  simple  materials  of  which  foods  are 
made.  These  foods  they  secure  either  directly  or  indirectly 
from  plants.  The  ability  of  plants  to  manufacture  com- 
plex foods  from  simple  substances  brings  up  several  ques- 
tions : 

What  is  the  method  by  which,  plants  produce  food  ?  Just 
what  parts  of  the  plants  do  the  work  ?  What  constitutes  the 
machinery  ?  Out  of  what  materials  is  the  food  manufactured  ? 
How  is  the  energy  supplied?  And  what  are  the  conditions 
under  which  the  process  goes  on  ? 

25 


26  Science  of  Plant  Life 

Photosynthesis.  The  primary  step  in  the  making  of  food 
is  the  building  of  carbohydrates  through  the  process  called 
photosynthesis  (Greek :  photos,  light,  and  synthesis,  putting 
together).  In  this  process  carbon  dioxid  from  the  air  and 
water  from  the  soil  are  brought  together  in  the  chloroplasts 
and  united  to  form  carbohydrates.  Sugar  is  the  first  abun- 
dant product,  but  being  soluble  in  the  water  of  the  cell,  it  is 
quite  invisible.  In  most  plants  a  large  part  of  the  sugar  is 
rapidly  changed  to  starch,  and  as  the  starch  is  insoluble  in 
water,  it  accumulates  temporarily  in  the  chloroplasts  in  the 
form  of  little  grains  which  may  readily  be  seen  with  a  mi- 
croscope. There  is  a  very  simple  test  for  the  presence  of 
starch.  A  solution  of  iodin  stains  most  substances  yellower 
brown,  but  it  stains  starch  blue  or  purple.  So  any  object 
that  contains  starch  —  a  cell,  a  leaf,  or  a  piece  of  cloth  —  will 
be  colored  purple  if  iodin  is  applied  to  it.  * 

Light  and  photosynthesis.  If  we  take  a  leaf  from  a  plant 
that  has  been  in  the  dark  for  two  days,  place  the  leaf  in  warm 
alcohol  to  remove  the  chlorophyll,  and  then  put  it  in  a  solution 
of  iodin,  it  is  stained  yellow.  This  proves  the  absence  of 
starch.  If  the  plant  is  then  put  in  the  light  for  a  few  hours, 
a  leaf  tested  in  the  same  way  will  be  colored  purple,  showing 
that  starch  is  present.  Evidently  light  is  necessary  for  photo- 
synthesis. It  is  not  surprising  to  find  that  light  is  so  effective 
in  building  up  compounds  in  the  green  parts  of  plants,  for  it 
is  a  powerful  agent  in  causing  chemical  changes.  You  may 
be  familiar  with  its  use  in  photography.  The  film  and  the 
printing  paper  have  on  them  a  layer  of  gelatin  containing 
certain  chemicals.  Exposure  to  the  light  for  even  a  fraction 
of  a  second  effects  in  these  chemically  treated  surfaces  changes 
which  may  be  seen  when  the  film  or  paper  is  developed. 


The  Manufacture  of  Food  27 

Many  chemical  substances  kept  in  drug  stores  must  be  pro- 
tected from  the  light;  otherwise  they  soon  change  their 
composition  and  become  different  substances. 

Chlorophyll  necessary  for  photosynthesis.  By  using  a 
plant  with  variegated  leaves,  the  iodin  test  will  show  that 
the  white  parts  form  no  starch.  Since  starch  is  formed  only 
in  the  green  part  of  the  blade,  it  is  evident  that  chlorophyll 
is  necessary  for  photosynthesis.  Any  green  part  of  a  plant  can 
carry  on  photosynthesis,  but  the  principal  food  factories  are 
the  leaves. 

Effects  of  temperature  on  photosynthesis.  The  effects  of 
temperature  on  photosynthesis  may  be  demonstrated  by  tak- 
ing plants  that  have  been  in  the  dark  long  enough  for  the 
starch  to  be  removed  from  the  leaves  and  placing  them  in 
the  light,  under  different  temperature  conditions.  Such  tests 
will  show  that  the  ordinary  summer  temperatures  are  most 
favorable  for  photosynthesis,  and  that  when  the  temperature 
falls  nearly  to  the  freezing  point  photosynthesis  decreases 
rapidly  or  ceases  entirely. 

Materials  and  products.  Experiments  have  shown  that 
the  materials  used  in  photosynthesis  are  carbon  dioxid  and 
waten  Carbon  dioxid  is  a  gas  that  makes  up  about  three  out 
of  every  10,000  parts  of  the  air.  Its  molecule  contains  one 
atom  of  carbon  and  two  atoms  of  oxygen  (CC^) .  Water,  which 
the  plant  gets  from  the  soil,  has  two  atoms  of  hydrogen  and 
one  atom  of  oxygen  in  every  molecule  (H2O).  The  carbo- 
hydrates made  in  photosynthesis  from  the  carbon  dioxid  and 
water  contain  these  same  elements.1  The  simple  sugars, 

1  Carbohydrates  include  many  substances  commonly  classified  as  sugars, 
starches,  and  celluloses.  The  simple  sugars,  glucose  and  fructose,  have  a 
formula  CeH^Oe.  The  double  sugars  like  sucrose  (cane  and  beet  sugar)  and 


28  Science  of  Plant  Life 

like  glucose,  which  are  the  first  abundant  products  of  photo- 
synthesis, contain  six  atoms  of  carbon,  twelve  atoms  of  hydro- 
gen, and  six  atoms  of  oxygen  in  each  molecule.  For  every 
molecule  of  glucose  manufactured,  therefore,  it  would  require 
six  molecules  of  carbon  dioxid  to  furnish  the  carbon  and  six 
molecules  of  water  to  provide  the  hydrogen.  These  amounts 
of  water  and  carbon  dioxid,  however,  contain  eighteen  atoms  of 
oxygen,  twelve  more  than  are  needed  for  the  making  of  glucose, 

6CO2+6H2O  =  C6Hi2O6+i2O. 

We  should  therefore  expect  oxygen  to  be  given  off  from 
leaves  during  photosynthesis.  That  this  actually  happens 
may  easily  be  shown  by  inverting  under  water  a  bundle  of 
the  branches  of  some  water  plant,  like  Elodea,  with  the  cut 
ends  placed  under  the  mouth  of  a  test  tube  that  is  filled  with 
water  (Fig.  18).  When  exposed  to  the  light  for  a  day,  the 
tube  will  be  partly  filled  with  gas.  By  testing  with  a  glowing 
match  or  splinter,  the  gas  may  be  shown  to  be  mostly  oxygen.1 

maltose  (malt  sugar)  may  be  built  up  by  combining  two  simple  sugars, 


glucose     fructose       cane  sugar  water 

one  molecule  of  water  being  lost  in  the  process  ;  and  they  may  be  split  up  into 
two  of  the  simple  sugars  by  the  addition  of  a  molecule  of  water.  The  starches 
and  celluloses  are  formed  by  combining  many  molecules  of  sugar  and  removing 
as  many  molecules  of  water  as  there  are  molecules  of  sugar  used.  Conse- 
quently their  formulas  are  (C6HioO5)n,  in  which  n  represents  a  rather  large 
number.  The  starches  and  celluloses  may  also  be  split  up  into  simple  sugars 
by  adding  the  required  number  of  molecules  of  water.  This  last  process  is 
the  one  by  which  corn  sirup  (glucose)  is  made  from  corn  starch.  The  process 
may  be  represented  by  the  equation, 

(C6Hi005)n  +  »(H20)  ->»(C6Hi206). 

starch  water  glucose 

1  Water  containing  a  considerable  amount  of  dissolved  carbon  dioxid  should 
be  used  in  this  experiment,  so  that  photosynthesis  may  go  on  rapidly.  For 
this  reason  pond  water  is  better  than  tap  water. 


The  Manufacture  of  Food 


29 


How  the  supplies  are  obtained.  Every  industrial  workshop 
must  constantly  be  provided  with  the  raw  materials  needed 
in  the  manufacture  of  its  prod- 
uct. Likewise  the  leaf  must  be 
supplied  with  the  substances 
that  it  uses  in  the  making  of 
food.  These  necessary  supplies 
come  to  the  leaf  through  the 
veins  and  the  stomata.  The 
water  passes  into  the  leaf 
through  the  water-conducting 
tissue  of  the  veins.  The  supply 
of  carbon  dioxid  reaches  the 
cells  of  the  mesophyll  through 
the  stomata  and  the  intercel- 
lular spaces.  When  the  stomata 
are  closed,  very  little  carbon 
dioxid  can  enter,  and  at  such 
times  the  process  of  photosyn- 
thesis is  of  necessity  greatly 
retarded. 

How  the  products  and  wastes 
are  removed.  The  manufac- 
ture of  carbohydrates  in  the  FlG- l8-  Experiment  to  show  the  giv- 

.  ing  off  of  oxygen  from  a  water  plant 

leaf    goes   on   only   during   the   (Eiodea)  during  photosynthesis. 
hours  of  sunlight ;    the  removal 

of  food  goes  on  at  all  times.  The  food-conducting  tissue  of 
the  veins  furnishes  the  outlet  for  the  product,  which  is  trans- 
ferred in  the  form  of  sugar.  During  the  day  the  rate  of 
manufacture  is  so  much  greater  than  the  rate  of  removal 
of  food,  that  starch  and  sugar  accumulate.  During  the 


Science  of  Plant  Life 


night  the  movement  of  food  into  the  stem  nearly  empties 
the  leaf,  and  by  early  morning  the  cells  are  again  in  good 

condition  for  food  manu- 
facture. The  waste  prod- 
uct, oxygen,  passes  from 
the  cells  to  the  intercellu- 
lar spaces  and  out  through 
the  stomata  to  the  atmos- 
phere. 

A  leaf,  then,  is  carrying 
on   photosynthesis    at    its 

A  maple  leaf  and  the  sugar  and  maple  fuil  Capacity  Only  when 
sirup  equivalent  to  the  amount  it  could  manu-  there  is  Sunlight,  a  f  avor- 
facture  in  a  season.  All  drawn  to  the  same  able  temperature,  and  an 

abundant    water     supply, 

and  when  the  stomata  are  open.  Even  under  these  condi- 
tions the  work  may  be  interfered  with  if  more  than  a  certain 
amount  of  the  products  accumulate  in  the  cells. 

The  amount  of  the  product.  The  amount  of  carbohydrates 
produced  in  photosynthesis  varies  so  greatly  in  different  plants 
and  under  dissimilar  conditions  that  it  is  very  difficult  to 
make  a  general  estimate  of  it.  The  result  of  many  experi- 
ments shows  that  under  favorable  conditions  a  square  meter 
of  leaf  surface  makes  on  an  average  about  i  gram  of  car- 
bohydrate per  hour.  At  this  rate  a  square  meter  of  leaf  sur- 
face in  midsummer  would  require  2  months  to  produce  food 
equivalent  to  that  consumed  by  the  average  man  in  a  day. 
This  average  rate  of  carbohydrate  manufacture  may  also  be 
expressed  by  saying  that  the  leaf  makes  enough  sugar 
in  a  summer  to  cover  it  with  a  layer  i  millimeter  thick 
(Fig.  19). 


The  Manufacture  of  Food 


Summary  of  photosynthesis.  We  may  summarize  the  facts 
we  have  learned  regarding  photosynthesis  by  likening  it  to 
a  manufacturing  process  of  human  invention : 


The  factory 

The  workrooms 
The  machinery 

The  energy 

The  raw  materials 

The  supply  department 


The  products 

The  forwarding  department 
The  waste  material 

The  working  hours 


is   the  green  tissue, .  especially 
that  of  the  leaves, 
are  the  cells.   . 

is    the    chloroplasts    and    the 
chlorophyll, 
is  the  sunlight. 

are  the  carbon  dioxid  and  water 
(C02andH20). 

is  the  stomata  and  intercellular 
spaces,  and  the  water-conduct- 
ing tissue. 

are  carbohydrates :  sugars 
(C6Hi2O6)  and  starches 
(C6H1005)n. 

is  the  food-conducting  tissue, 
and  it  works  both  day  and  night, 
is  oxygen,  which  escapes  through 
the  intercellular  spaces  and  the 
stomata. 
are  all  the  hours  of  sunlight. 


The  production  of  fats.  In  addition  to  carbohydrates, 
plants  make  and  use  two  other  classes  of  foods :  fats  and 
proteins.  The  fats  are  quite  similar  to  the  carbohydrates  in 
composition.  They  contain  the  same  chemical  elements : 
carbon^  hydrogen^  and  oxygea.  The  proportion  of  the 
oxygen  to  carbon,  however,  is  smaller.  At  ordinary  temper- 
atures fats  occur  in  plants  both  as  solids  and  liquids.  The 


32  Science  of  Plant  Life 

liquid  fats  are  commonly  called  oils.  They  are  probably 
made  directly  from  the  carbohydrates.  As  there  appears 
to  be  present  in  the  cell  no  special  fat-producing  apparatus 
for  bringing  about  this  chemical  change,  it  is  probably  effected 
by  the  protoplasm  and  fat  can  therefore  be  formed  in  any 
living  part  of  the  plant.  Although  fats  are  widely  distributed 
in  the  plant  body,  they  are  especially  abundant  in  seeds  and 
fruits.  Some  of  the  commonest  fats  and  oils  of  commerce 
derived  from  plants  are  corn  oil,  coconut  oil,  cottonseed  oil, 
linseed  oil,  castor  oil,  olive  oil,  peanut  oil,  and  cocoa  butter. 

The  making  and  use  of  proteins.  The  proteins  are  the  third 
class  of  foods.  They  too  are  constructed  in  large  part  from 
the  carbohydrates ;  but  their  molecules  are  vastly  more  com- 
plex than  are  the  molecules  of  carbohydrates  and  fats,  and, 
in  addition  to  carbon,  hydrogen,  and  oxygen,  they  contain 
the  elements  nitrogen  and  sulfur,  and  occasionally  phos- 
phorus. In  protein  synthesis  the  amount  of  sulfur  and 
phosphorus  consumed  is  small,  but  a  very  large  amount  of 
nitrogen  is  required.  Furthermore,  nitrogen  in  the  gaseous 
condition  in  which  it  occurs  in  the  air  does  not  readily  unite 
with  other  substances ;  so,  although  it  makes  up  four  fifths 
of  the  atmosphere,  green  plants  cannot  take  it  directly  from 
the  air.  For  the  nitrogen  needed  for  protein-making  they 
must  depend,  therefore,  on  the  supply  which  comes  from  the 
soil  in  the  form  of  nitrates.  This  is  carried  to  the  cells 
with  the  water  that  is  absorbed  by  the  roots.  Protein  syn- 
thesis, like  the  synthesis  of  fats,  is  probably  effected  by  the 
protoplasm.  It  may  occur  in  nearly  all  parts  of  a  plant,  but 
it  takes  place  for  the  most  part  in  the  leaves.  Proteins,  be- 
cause of  their  complex  composition,  are  especially  used  in 
building  up  and  repairing  the  protoplasm.  They  are  trans- 


The  Manufacture  of  Food  33 

ported  from  the  leaves  by  the  food-conducting  tissue  of  the 
bundles. 


Soy  beans 


Wheatflour 

Corn  meal  ., 

ice 


175      127      97       T         6.8 

FIG.  20.     Percentage  of  protein  in  various  foods. 

The  most  expensive  portion  of  the  diet  of  human  beings  is 
the  proteins.  Figure  20  shows  that  in  soy  beans  we  possess 
the  richest  source  of  protein.  It  also  shows  why  the  soy 
bean  is  one  of  the  most  important  of  foods  in  the  Asiatic 
nations,  where  animal  foods  are  very  limited.  One  dollar 
will  buy  several  times  as  much  protein  in  soy  beans  as  it 
will  in  any  other  plant  or  animal  food.  However,  recent  ex- 
periments in  animal  feeding  have  shown  that  for  mainte- 
nance and  growth  some  proteins  are  more  valuable  pound 
for  pound  than  others. 

Amount  of  food  produced  per  acre.  Since  the  food  supply 
of  all  living  beings  depends  primarily  upon  these  synthetic 
processes  that  are  carried  on  in  plants,  it  is  of  interest  to 
inquire  how  much  food  may  be  derived  from  an  average  acre 
of  land  when  planted  to  different  crops.  It  must  be  remem- 


34 


Science  of  Plant  Life 


bered  that  the  plants  that  produce  this  food  take  a  consider- 
able part  for  their  own  maintenance,  and  that  the  part  which 
the  farmer  harvests  is  the  plants'  surplus.  The  table  given 
below  shows  the  average  yield  per  acre;  its  food  value  cal- 
culated in  Calories ; l  and  the  number  of  men  that  i  acre 
planted  to  different  crops  might  feed  for  i  day,  assuming  that 
each  man  requires  3000  Calories  per  day. 


YIELD  PER 

ACRE 

MILLIONS  OF 

NUMBER  OF  MEN 

Bushels 

Pounds 

EQUIVALENT 

FED  FOR  i  DA* 

Corn  .  . 
Sweet  potatoes  .  .  .  . 
Irish  potatoes  .  .  .  . 
Wheat 

35 
no 

IOO 
20 

1960 
5940 
6000 

I  2OO 

3-1 

2.8 

•9 
8 

IOOO 

900 
600 
600 

Rice  
Soy  beans  ....  .  - 
Beans  

40 

16 
14 

H54 
960 
840 

•  7 
•5 
.1 

56o 
500 
375 

If  the  plant  products  of  an  average  acre  are  fed  to  cattle, 
the  dressed  beef  produced  amounts  to  only  125  pounds, 
yielding  an  energy  equivalent  to  the  food  of  43  men  for  i  day. 
If  transformed  into  pork,  the  yield  is  273  pounds,  or  sufficient 
food  for  220  men  for  i  day.2  This  shows  the  great  loss  of 
energy  that  results  when  plant  foods  are  converted  into  meat 
before  they  reach  the  human  consumer.  It  is  evident  that  as 
the  human  family  becomes  larger  and  food  becomes  scarcer, 
we  shall  have  to  take  more  and  more  of  our  foods  directly 
from  plants. 

1  A  Calorie  is  the  amount  of  heat  necessary  to  raise  the  temperature  of  i  kilo 
of  water  i  degree  Centigrade. 

*  United  States  Department  of  Agriculture,  Farmers'  Bulletin  No.  877. 


The  Manufacture  of  Food  35 

PROBLEMS 

1.  How  do  the  white  parts  of  a  variegated  leaf  get  food? 

2.  Occasionally  in  a  field  of  young  corn  a  stalk  that  lacks  chlorophyll  will  be 
found.     This  white  stalk  dies  as  soon  as  the  food  supply  in  the  grain  from 

which  it  came  is  exhausted.     Why? 

3.  Geraniums  with  variegated  leaves  occasionally  produce  branches  that  are 
entirely  white.     A  noted  horticultural  firm  offered  $1000  to  any  one  of  its 
gardeners  who  would  root  one  of  these  branches  and  thus  produce  a  white- 
leafed  geranium.     What  was  the  chance  for  success  ?     Why  ? 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Four 

1.  Examine  one  or  more  plants  from  each  of  the  following  groups, 
noting  the  arrangement  of  leaves  on  vertical  and  inclined  stems : 

Corn,  elm,  hackberry,  wheat,  rye,  canna,  bamboo. 
Lilac,  viburnum,  catalpa,  ash,  maple. 
Willow,  poplar,  pine,  spruce,  peach,  apple,  sunflower. 
Cat-tail,  iris,  onion,  hyacinth,  yucca,  century  plant. 

2.  In  the  field,  study  some  of  the  following  plants,  noting  the 
positions  their  leaves  take  with  reference  to  light : 

Sugar  maple,   elm,   beech,   oak,   catalpa,   Virginia  creeper, 

Boston  ivy. 

Tulip  tree,  cot tonwood,  birch,  willow,  spirea,  barberry. 
Which  cast  the  more  complete  shade,  trees  whose  leaves  are 
sensitive  or  insensitive  to  light? 

3.  On  several  of  the  following  plants  study  the  leaf  positions,  to 
determine  how  the  blades  reach  their  positions  when  mature : 

Sunflower,  mallow,  Virginia  creeper,  Boston  ivy,  English 
ivy,  wild  prickly  lettuce,  nasturtium,  ragweed,  compass 
plant,  water  lily.  Other  plants  of  the  locality  will  furnish 
additional  material  for  study. 

4.  Examine  submerged  plants  like  pondweeds,  Elodea,   Cera- 
tophyllum,  and  water  buttercups,  and  compare  their  leaves  with 
those  of  land  plants. 

5.  Sections  of  leaves  of  different  kinds  may  be  used  to  show  vari- 
ations in  internal  structures. 

6.  Experiments : 

Grow  geraniums  and  nasturtiums  with  one-sided  illumination. 
Mark  the  original  position  of  the  leaves  with  stakes  and  note 
changes  in  position. 

Try  the  effects  of  light  and  darkness  on  the  positions  of  leaves  of 
sensitive  plants,  beans,  or  sweet  clover.  Black  paper  covers  may 
be  used  to  exclude  the  light. 


CHAPTER   FOUR 


Note  the  alternate 

arrangement  of  the  leaves. 


LEAVES  IN  RELATION  TO  LIGHT 

THE  leaf,  as  we  have  seen,  must  receive  light  in  order  to 

produce  food.     Leaves  are  variously  arranged  on  stems,  and 

stems    have   all    sorts  of    positions. 

Many  of  these  leaf  arrangements  and 

stem  positions  are  not  advantageous 

for  the  display  of  leaves  to  the  light. 

The  leaf,  however,  and  especially  the 

petiole,  is  so  influenced  by  light  dur- 
ing  its   development,  that   the   leaf 

when  mature  has  the  best  possible 

position  with  respect  to  light.     The 

raised   leaves  of    the   pumpkin,   the  FIG.  21.    Vertical  branch  of 

mosaics  of  leaves  formed  on  the  sides 

of  buildings  by  the*  Boston  ivy,  and 

the   successive  tiers  of   leaves  on   a   beech    tree   illustrate 

different  arrangements  by  which  large  numbers  of    leaves 

are  efficiently  displayed  to  the  light. 
The  arrangement  of  leaves  on  stems.     Leaves  develop 

from  somewhat  thickened  places  on  the  stems,  called  the 

nodes.  Each  node 
may  bear  one,  two, 
or  several  leaves. 
According  to  the 
number  of  leaves 
that  the  node 
bears,  the  leaf  ar- 
rangement is  desig- 

FIG.  22.     Horizontal  branch  of  magnolia.     Compare  leaf 

positions  with  those  of  Figure  21.  Opposite, 

37 


Science  of  Plant  Life 


In  the  alternate  arrangement  each  node  bears  one  leaf. 
This  is  also  spoken  of  as  the  spiral  arrangement,  because  a 
line  drawn  through  successive  leaf  bases 
forms  a  spiral  about  the  stem.  Some- 
times, as  in  the  corn  plant,  the  spiral 
passes  half  around  the  stem  in  going  from 
one  node  to  the  next  (page  50).  In  other 
plants,  like  the  sedges,  the  spiral  passes 
but  a  third  around  the  stem  between 
nodes.  In  several  of  our  common  fruit 
trees,  as  the  apple  and  the  peach,  the 
spiral  between  nodes  passes  two  fifths 
around  the  stem.  These  variations  of 
the  spiral  arrangement  are  called  the  two- 
ranked  (Fig.  28),  three-ranked  (Fig.  23), 
and  five-ranked 
arrangements. 

F,G.23.     Asedge(D»«-         I*  the  Opposite 

chiwn),  showing  three-  arrangement  two 

ranked  arrangement  of  leayes  QCCUr  at 
the  leaves. 

each  node.     The 

leaves  at  successive  nodes,  how- 
ever, are  at  right  angles  to  each 
other  (Fig.  25),  giving  four  ranks 
of  leaves.  The  maple,  ash,  dog- 
wood, and  lilac  furnish  examples 
of  the  opposite  arrangement.  In 
the  whorled  arrangement  the 
leaves  are  in  a  circle  about  the 

.        T     ..  ,  FIG.   24.     Indian  cucumber  root, 

node.     The  Indian  cucumber  root   showin4g  the  whorled  arrangement 
(Medeola)     and     the     wood     lily  of  the  leaves. 


Leaves  in  Relation  to  Light 


39 


(page  310)  furnish  excellent  examples  of  the  whorled  arrange- 
ment. 

However,  it  is  only  on  upright 
stems  which  receive  the  light  equally 
on  all  sides,  that  the  blades  take 
their  normal  positions  directly  out 
from  the  nodes.  If  an  erect  shoot 
be  placed  in  an  inclined  position,  it 
is  easy  to  see  that  the  leaves  are  no 
longer  well  displayed  to  the  light. 
As  may  be  readily  seen  by  examin- 
ing the  branches  of  trees  and  the 
stems  of  trailing  plants,  horizontal 
or  inclined  stems  become  twisted 
during  development  because  of  un- 
equal illumination  (Fig.  26).  The 
twisting  of  the  stems  brings  the 
leaves  into  better  positions  to  re-  FlG- 25-  Vertical  branch  of  dog- 

.  .  wood,  showing   the  opposite  ar- 

CClVe  hght,  but  it  often  obscures  the    rangement  of  the  leaves. 

normal  arrangement  of  the  leaves. 

The  positions  of  leaves  with  reference  to  light.     If  leaves 
are  moderately  sensitive   to  light,   they  assume  a  position 


FIG.  26.     Horizontal  branch  of  dogwood.     Compare  with  Figure  25. 


Science  of  Plant  Life 


approximately  at  right  angles  to  the  line  along  which  the 
greatest  amount  of  light  reaches  them.  Consequently  the 
leaves  on  most  of  our  common  trees,  shrubs,  and  herbs  tend  to 
take  an  approximately  horizontal  position.  The  sugar  maple 
and  the  horse-chestnut  are  examples  of  trees  whose  leaves 
are  displayed  in  this  manner.  In  the  cottonwood  and  tulip 
tree  the  leaves  are  only  slightly  sensitive  to  light,  and  the 
result  is  that  their  leaves  assume  a  great  variety  of  positions. 

If  leaves  are  extremely  sensi- 
tive to  light,  the  blades  may 
turn  toward  the  sun  in  the 
early  morning  and  follow  the 
sun  throughout  the  day,  always 
keeping  the  broad  face  of  the 
leaf  to  the  light.  The  leaves 
of  the  common  mallow  move 
in  this  way. 

Compass  plants.  There  is 
another  class  of  plants  which 
are  sensitive  to  light,  but 
which  respond  to  it  in  a  very 
different  manner.  These  are 
the  so-called  compass  plants, 
of  which  the  wild  prickly  let- 
tuce is  a  widely  distributed 
example.  In  sunny  situations 
the  leaves  of  these  plants  tend 
to  take  positions  edgewise  to 
FIG.  27.  Prickly  lettuce  plant:  4,  viewed  the  direction  of  the  most  in- 

from  west;  B,  viewed  from  south.  Drawn  ^^Q  j-  ht  As  ^  sumight 
from  a  specimen  grown  under  exposure  .  . 

to  bright  sunlight.  is  most  intense  at  noon,  it  is 


Leaves  in  Relation  to  Light 


only  in  the  morning  and  late  afternoon  that  the  flat  sides  of 
the  leaves  are  perpendicular  to  the  sun's  rays.  This  response 
to  the  light  also  places  most  of  the  leaves  in  a  vertical  north- 
and-south  plane  and  suggests  the  name  "  compass  plant." 
When  grown  in  partial  shade,  the  leaves  of  these  same  plants 
are  horizontal.  Hence  it  is  clear  that  the  position  of  their 
leaves  in  sunny  situations  is  the  result  of  light  conditions. 

How  the  blade  attains  its  position  with  reference  to  the  light. 
The  attainment  of  its  position  by  the  leaf  blade  is  partly  ac- 
complished, as  has  been  noted,  by  the  bending  and  twisting 
of  the  plant  stem  during  its  development.  To  a  much  greater 
extent  the  blade  owes  its  position  to  the  bending,  twisting,  and 
elongating  of  the  petiole.  In- 
deed, this  is  the  particular  ad- 
vantage of  the  petiole.  Its 
length  and  direction  of  growth 
are  for  the  most  part  deter- 
mined by  the  way  in  which  the 
light  falls  on  the  blade  during 
the  period  of  development.  An 
examination  of  a  branch  of  a 
maple  will  disclose  how  the 
lengthening  and  bending  of  the 
petioles  help  to  fit  each  leaf 
into  a  position  where  it  will 
receive  the  light. 

Vertical  leaves.  In  a  number 
of  common  plants,  including 
the  iris,  cat- tail,  calamus,  and 
many  grasses,  the  leaves  are  , 

/     °  FIG.  28.     Ins,  showing  leaves  held  in 

Vertical    because    they   are    held     vertical  position  by  the  sheathing  bases. 


Science  of  Plant  Life 


FIG.  29.     Guinea  grass,  a  plant  grown  in  the  tropics  for  fodder.     Note  the  vertical 
leaves  and  the  large  amount  of  leaf  surface  exposed  by  the  plant  to  the  light. 

in  this  position  by  their  sheathing  bases  rather  than  because 
of  a  response  to  light.  These  plants  usually  occur  in  dense 
growths,  and  the  vertical  position  of  the  leaves  permits  the 
light  to  penetrate  to  their  bases.  This  has  the  advantage  of 
allowing  photosynthesis  to  go  on  throughout  the  entire 
length  of  the  leaves. 

Differences  in  vertical  and  horizontal  leaves.  Vertical 
leaves  differ  from  horizontal  leaves  in  several  particulars : 

In  vertical  leaves  the  mesophyll  may  be  composed  of 
spongy  tissue,  or  it  may  be  composed  entirely  of  palisade 
cells.  More  rarely  there  are  palisade  layers  on  both  sides, 
with  a  spongy  layer  between.  In  contrast,  a  horizontal  leaf 
usually  has  a  palisade  layer  beneath  the  upper  epidermis,  and 
the  lower  portion  of  the  mesophyll  is  composed  of  loosely 
arranged  cells.  In  vertical  leaves  stomata  usually  occur  on 


Leaves  in  Relation  to  Light  43 

both  surfaces,  while  in  most  horizontal  leaves  the  stomata 
are  confined  to  the  lower  surface  (page  18).     Vertical  leaves 


FiG.  30.     Various  positions  taken  by  leaflets  of  lima  bean :  A ,  position  in  intense 
light ;  B,  position  in  diffuse  light ;   C,  position  in  darkness. 

are  likely  to  be  of  the  same  color  on  both  surfaces,  while 
horizontal  leaves  are  generally  of  a  darker  green  on  the  upper 
surface. 

The  difference  in  the  color  of  the  two  sides  of  a  horizontal 
leaf  is  due  to  the  presence  of  a  larger  amount  of  chlorophyll 
in  the  compact  palisade  layers  of  the  mesophyll  than  in  the 
loose  spongy  layers  beneath.  In  vertical  leaves,  the  similarity 
of  structure  in  the  mesophyll  on  each  side,  and  the  fact  that 
both  surfaces  of  the  leaf  are  equally  illuminated,  account  for 
the  sameness  of  color  of  the  two  surfaces. 

Motile  leaves.  The  leaves  of  which  we  have  been  speaking 
have  their  positions  rather  definitely  fixed  when  they  reach 
maturity.  There  is  another  class  of  leaves,  however,  in  which 
the  positions  of  the  blades  are  not  fixed  but  are  changed  ac- 
cording to  the  intensity  and  direction  of  light.  A  familiar 
example  is  the  roadside  sweet  clover.  At  night  the  three 


44 


Science  of  Plant  Life 


leaflets  of  the  compound  leaf  droop  downward  from  the  peti- 
ole ;  in  the  medium  light  of  a  cloudy  day  they  are  held  per- 
pendicular to  the  light;  in 
the  most  intense  sunlight 
the  blades  are  raised  above 
the  petiole  until  they  are 
edgewise  and  point  toward 
the  light.  Some  observa- 
tion of  bean  seedlings,  which 
may  readily  be  grown  in 
the  laboratory,  will  be  in- 
structive in  this  connection. 
Other  examples  of  motile 
leaves  may  be  seen  in  the 
honey  locust,  the  leaflets  of 
which  fold  upward  at  night, 
and  in  white  clover,  oxalis, 
and  the  red-bud  tree.  The 
leaflets  of  the  sensitive 
plant  vary  their  positions 
according  to  light  intensity, 
and  also  when  touched  or 


FIG.  31.  Sensitive  plant.  The  leaves  on  the 
left  side  are  in  normal  positions ;  those  on  the 
right  side  have  been  touched  and  the  leaflets 
have  folded  together  wholly  or  in  part  and 
the  petioles  have  folded  toward  the  stem. 
P  is  the  pulvinus. 


injured   in   any  way   (Fig. 

31). 

The  change  of  position  in 
motile  leaves  is  brought 
about  by  changes  in  the  water  content  of  the  cells  on  oppo- 
site sides  of  a  special  organ  called  the  pulvinus  (Fig.  32), 
which  is  located  at  the  base  of  the  leaflets.  This  device  may 
readily  be  studied  in  the  leaf  of  the  bean. 
The  leaves  of  shade  plants.  As  may  be  observed  by  a  trip 


Leaves  in  Relation  to  Light 


45 


to  the  woods,  the  leaves  of  plants  growing  in  the  shade  are 
usually  darker  and  more  bluish-green  than  the  leaves  of 
plants  growing  in  full  sunlight. 
This  difference  in  color  is  ac- 
counted for  in  part  by  the 
amount  of  chlorophyll  near  the 
surface  and  in  part  by  a  slight 
difference  in  the  color  of  the 
chlorophyll  itself  (page  63). 
In  a  few  shade  plants  the  depth 
of  the  green  color  is  increased 
by  the  presence  of  chloroplasts 
in  the  epidermal  'cells.  Shade 
plants  are  not  subjected  to 
drying,  as  are  plants  growing 
in  exposed  situations,  and  gen-  FlG  32  Pulvinus  and  section  of  pul. 

erally  Speaking  their  leaves  are    vinus  from  leaf  of  sensitive  plant,  both 

broad  and  thin.  The  leaves 
of  these  plants  differ  further 
from  the  ordinary  leaf  in  that  causins  the  cells  Partially  to  coIlaPse- 

The  pressure  of  the  cells  on  the  side  B 
the    CUtlde     IS     leSS    developed,     then  forces  the  leaf  downward. 

the  mesophyll  is  composed  al- 
most  entirely   of   spongy   tissue,  and   usually  stomata  are 
present  on  both  surfaces  of  the  leaf. 

Submerged  leaves.  Every  one  who  has  gone  fishing  or 
rowing  knows  that  a  great  deal  of  sunlight  is  reflected  from  the 
surface  of  water.  This  means  that  the  amount  of  light  that 
penetrates  the  surface  is  reduced  by  just  the  amount  that  is 
reflected.  The  penetration  of  the  water  by  the  sun's  rays  is 
further  interfered  with  by  the  fine  sediment  that  clouds  our 
ponds  and  lakes.  Every  one  who  has  dived  and  opened  his 


enlarged.  When  the  leaf  is  touched,  the 
water  in  the  cells  on  the  side  A  passes 
outward  into  the  intercellular  spaces, 


46  Science  of  Plant  Life 

eyes  under  water  knows  that  it  is  dark  at  a  comparatively 
slight  depth.  Hence  submerged  plants  always  grow  in  light 
of  reduced  intensity.  They  receive  an  amount  of  light  com- 
parable to  that  received  by  the  shade  plants  found  in  forest 
ravines.  Submerged  leaves,  too,  are  of  very  soft  texture, 
and  are  quite  without  mechanical  tissue  in  the  veins,  so  that 
they  are  unable  to  support  themselves  when  lifted  from  the 
water.  They  are  kept  upright  in  the  water  by  their  buoyancy, 
which  is  due  to  the  greatly  enlarged  air  spaces  among  the 
mesophyll  cells. 

Summary.  Light  has  a  marked  effect  upon  the  positions, 
the  color,  and  the  structures  of  leaves.  Leaves  tend  to  be 
placed  directly  outward  from  the  nodes  to  which  they  are 
attached,  but  light  affects  them  during  their  development, 
and  most  leaves  come  to  occupy  positions  that  have  more 
relation  to  the  light  than  to  the  stem  which  bears  them.  Some 
leaves  vary  their  positions  constantly  as  the  light,  changes. 

Leaves  of  plants  grown  in  weak  light,  including  those  of 
submerged  plants,  usually  have  the  mesophyll  made  up  wholly 
or  largely  of  spongy  cells.  Leaves  of  plants  grown  in  intense 
light  usually  have  the  mesophyll  made  up  wholly  or  largely 
of  palisade  cells. 

PROBLEMS 

1.  Why  do  house  plants  flourish  best  at  south  windows  in  the  winter  time  ? 

2.  What  part  of  full  sunlight  is  received  by  a  plant  that  stands  near  a  window? 

3.  Why  do  gardeners  shade  lettuce  plants  in  midsummer? 

4.  What  other  condition,  besides  light  intensity,  is  affected  by  shading  ? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Five 

1.  Cut  a  leaf  from  some  plant,  preferably  under  water.     Then 
immerse  the  cut  end  in  red  ink  or  eosin  and  let  it  stand  for  some 
time,  watching  the  movement  of  the  red  color  in  the  veins.     Thin 
leaves,  like  those  of  balsam  and  nasturtium,  are  especially  suited 
to  this  experiment.     If  thick  leaves  are  used,  they  may  be  split 
with  a  knife  in  order  to  show  the  veins. 

2.  Study  the  surface  coverings  —  cuticle,  wax,  and  hairs  —  on 
a  series  of  leaves  like  the  mullein,  century  plant,  bur  oak,  tomato, 
and  geranium. 

3.  Compare  leaves  of  water  plants  and  moist  soil  plants  with 
those  of  desert  forms  like  the  yucca,  agave,  and  cactus. 

4.  Examine  free-hand  sections  of  leaves  or  prepared  microscope 
slides  that  will  show  variations  in  stomata,  compactness  of  tissue, 
development  of  air  chambers,  and  water  storage. 

5.  Determine  the  relative  time  required  for  the  wilting  and 
complete  drying  out  of  several  kinds  of  leaves,  using  leaves  of 
different  texture,  thickness,  and  surface  covering. 

6.  Take  three  similar  leaves  from  a  single  plant.     Cover  with  a 
thin  layer  of  vaselin  the  upper  surface  of  one,  the  lower  surface  of 
another,  and  both  surfaces  of  the  third.     Determine  the  relative 
time  required  for  the  leaves  to  wilt  and  also  to  dry  out  completely. 

7.  Experiments : 

Show  that  water  is  given  off  by  plants,  by  covering  a  plant  with 
a  bell  jar. 

Ascertain  the  amount  of  water  transpired  by  a  potted  plant,  by 
covering  the  pot  and  soil  with  melted  paraffin  and  weighing  the 
plant  at  intervals. 


47 


CHAPTER   FIVE 

THE   WATER   RELATIONS    OF  LEAVES 

DURING  a  prolonged  drought  in  Illinois,  in  1914,  oats  in  some 
places  failed  to  attain  a  height  of  more  than  4  inches  and 
produced  practically  no  grain,  and  corn  which  should  have 
averaged  10  feet  in  height  reached  only  5  feet  in  many  fields, 
and  yielded  only  half  the  normal  amount  of  grain.  In  the 
four  great  corn-growing  states  there  must  be  3  inches  of 
rainfall  in  July  for  the  best  yield  of  corn ;  and  if  the  rainfall 
during  July  is  2-J  inches  instead  of  3,  it  is  estimated  that  at 
normal  prices  there  is  an  average  loss  of  $5  an  acre,  or  a  total 
loss  of  $150,000,000.  Those  who  cultivate  plants  know  from 
experience  the  importance  of  a  sufficient  water  supply  in  the 
production  of  crops,  and  the  reason  why  the  water  supply  is 
important  will  be  apparent  when  we  understand  the  uses 
made  of  water  by  the  plant. 

Why  water  is  necessary  to  a  plant.  The  active  protoplasm 
of  all  plant  cells  is  in  a  semiliquid  condition.  More  than 
90  per  cent  of  its  weight  is  made  up  of  water,  and  in  consist- 
ency it  closely  resembles  white  of  egg.  The  several  parts  of 
the  protoplasm  —  the  cytoplasm,  the  nucleus,  and  the  plas- 
tids  —  differ  somewhat  in  their  water  content,  but  all  of 
them  must  be  nearly  saturated  with  water  to  carry  on  the 
life  processes.  When  the  amount  of  water  in  the  cell 
falls  much  below  this  point,  the  protoplasm  becomes  rigid 
and  its  processes  are  retarded.  In  many  plants  the  pro- 
toplasm may  even  die  if  the  water  content  is  greatly 
reduced.  Water  is  necessary  for  the  life  of  the  protoplasm  of 
plant  cells. 

Water  is  one  of  the  materials  used  in  the  production-  of 
carbohydrate.  Without  it  the  process  of  photosynthesis, 

48 


The  Water  Relations  of  Leaves  49 

upon  which  the  world  depends  for  its  supply  of  food,  cannot 
be  carried  on.  Water  is  necessary  for  photosynthesis. 

Substances  can  enter  plants  only  when  they  are  in  solution. 
Both  the  gases  and  the  mineral  compounds  'that  are  used 
by  the  plant  in  its  various  processes  must  be  in  solution  in 
water  before  they  can  be  absorbed  or  pass  from  one  cell  to 
another  within  the  plant.  Indirectly  as  well  as  directly  water 
is  necessary  to  photosynthesis  ;  for  water  keeps  the  mesophyll 
cells  wet  and  thus  makes  it  possible  for  the  carbon  dioxid  to 
enter  the  cells.  Water  is  necessary  for  the  absorption' of  min- 
erals and  gases  and  for  the  transfer  of  materials  within  the  plant. 

Vacuoles  the  reservoirs  of  the  cells.  The  vacuoles  inside 
the  cytoplasm  are  minute  reservoirs  within  the  cells.  They 
contain  the  cell  sap,  which  consists  of  water  holding  in  solu- 
tion sugars,  mineral  salts,  and  acids.  The  relation  of  the 
vacuoles  to  the  protoplasm  is  most  important,  for  the  pro- 
toplasm can  secrete  excess  substances  into  the  vacuole  or 
remove  substances  from  it  as  they  are  needed.  In  many 
industrial  establishments  individual  machines  are  provided 
with  small  boxes  or  trays,  some  to  hold  raw  materials  and 
others  to  receive  the  manufactured  product.  The  vacuoles 
have  the  same  function  as  these  storage  boxes :  they  hold  a 
supply  of  the  raw  material  for  the  use  of  the  protoplasm,  and 
they  receive  some  of  the  products  that  result  from  the  ac- 
tivities within  the  cell. 

Transpiration.  If  we  expose  a  wet  cloth  to  the  air,  the 
water  evaporates ;  that  is,  it  changes  from  a  liquid  to  a 
vapor  and  passes  off  into  the  atmosphere.  The  same  thing 
happens  when  a  plant  is  exposed  to  the  air.  The  mesophyll 
cells  of  the  leaf  are  continually  giving  up  water  vapor  to  the 
intercellular  spaces,  from  which,  if  the  stomata  are  open, 


Science  of  Plant  Life 


this  vapor  passes  out  into  the  atmosphere.  The  epidermis  of 
the  leaf  also  allows  some  water  to  pass  through  it,  but  in  land 
plants  this  is  a  relatively  small  amount,  because  the  cuticle 
hinders  the  process  (page  54).  The  giving  off  of  water  vapor 

from  plants  is  called  transpiration. 

The  loss  of  water  in  the  form  of 
vapor  is  a  process  that  takes  place  in 
animals  as  well  as  in  plants.  If  you 
hold  your  hand  near  a  windowpane 
on  a  cool  day,  a  halo  of  minute  water 
drops  condenses  on  the  glass.  These 
water  particles  come  from  the  moist 
cells  of  your  skin.  If  you  blow  on 
a  glass,  water  collects  even  more 
abundantly.  The  vapor  in  the 
breath  is  water  that  has  evaporated 
from  the  moist  cells  of  the  lungs. 

The  amount  of  water  transpired 
by  plants.  The  amount  of  water 
given  off  in  transpiration  is  surpris- 
ingly large.  During  its  lifetime,  a 
well-  watered  corn  plant  may  give  off 
4  or  5  gallons  of  water.  A  sunflower 
plant  may  transpire  more  than  18 
gallons.  The  water  given  off  by  a 
FIG.  33.  Com  plant,  and  bottle  field  of  wheat  during  its  entire 

of  water  equivalent  to  that  tran-  period  of  development  WOUld  COVCr 
spired  by  the  plant  during  its  life-  ,,  ~  ,  ,  ,  ,  ,.  r  ., 

time.     (All  drawn  to  the  same     the  field  to  a  depth  of  4  Or   5  inches. 

scale.)     In  eastern  Colorado  the    For  the  best  growth  of  plants,  there- 


amount. 


soil  enough  water  to  permit  them  to 


The  Water  Relations  of  Leaves  51 

take  what  they  need  for  transpiration.  When  we  consider 
that  the  quantity  of  water  transpired  by  wheat  in  cultiva- 
tion is  one  fifth  to  one  eighth  of  the  rainfall  of  the  central 
United  States,  we  begin  to  realize  how  large  a  fraction  of 
all  the  water  that  falls  on  the  soil  is  actually  used  by  the 
plants.  In  all  rainfall,  some  water  runs  off  the  soil  without 
penetrating  the  surface,  some  evaporates  from  the  soil  surface 
itself,  and  some  sinks  below  the  level  of  the  plant  roots.  Con- 
sequently, it  is  only  when  there  are  abundant  rains,  dis- 
tributed throughout  the  growing  season,  that  the  amount 
of  water  needed  by  the  plants  for  their  best  development  is 
available  in  the  upper  layers  of  the  soil.  It  has  been  shown 
by  experiment  that  for  the  production  of  every  pound  of 
solid  matter  in  the  above-ground  parts  of  crop  plants,  from 
300  to  500  pounds  of  water  are  required  in  the  central  United 
States,  and  that  from  400  to  1000  pounds  are  needed  on  the 
plains  of  Colorado.  The  amount  of  water  used  in  transpi- 
ration is,  therefore,  many  times  the  amount  used  in  the  manu- 
facture of  food. 

Water  supply  and  crop  yields.  Knowing  these  water  re- 
quirements, it  is  easy  to  understand  why  droughts  are  so  dis- 
astrous to  crops.  When  the  rainfall  is  slight,  not  only  is  the 
amount  of  water  that  can  be  secured  by  the  plant  from  the 
soil  reduced,  but  the  sunshine  is  brighter  and  the  air  is 
usually  drier,  so  that  transpiration  from  the  plant  is  in- 
creased. It  is  in  part  because  of  the  water  requirement  of 
crop  plants  that  bottom  lands  —  lands  along  streams  in 
the  bottoms  of  valleys  —  are  more  valuable  for  growing 
crops  than  are  uplands.  There  the  underground  water  is 
nearer  the  surface,  and  keeps  the  supply  for  plants  more 
nearly  constant. 


52  Science  of  Plant  Life 

The  balance  between  transpiration  and  absorption.     The 

amount  of  water  in  the  cells  of  the  plant  as  a  whole  is  de- 
termined largely  by  two  processes  :  (i)  the  rate  of  absorption 
-  the  taking  of  water  from  the  soil ;  and  (2)  the  rate  of 
transpiration.  The  relation  between  these  two  rates  de- 
termines the  water  balance  inside  the  plant.  If  the  tran- 
spiration is  rapid  and  absorption  is  slow,  internal  drought 
results  and  the  plant  may  wilt.  If  the  transpiration  is  slow 
and  the  water  intake  is  rapid,  the  cells  will  be  filled  to  their 
utmost  capacity. 

Importance  of  the  water  balance.  Of  all  the  factors  that 
influence  the  growth  of  plants  and  modify  the  form,  size,  and 
structure  of  leaves,  the  water  content  of  the  cells  is  the  most 
important.  Abundant  water  permits  a  plant  to  grow  to  its 
greatest  height,  and  permits  the  leaves  to  attain  their  largest 
size  and  number.  Long-continued  internal  drought  may 
cause  the  plant  to  be  dwarfed  and  the  leaves  to  be  small  and 
few  in  number.  In  the  river  bottom  the  bur  oak  may  develop 
into  a  magnificent  tree  100  feet  in  height,  while  on  the  dry 
uplands  it  may  attain  only  a  stunted  growth  of  less  than  15 
feet.  An  average  leaf  on  a  large  tree  will  have  twice  the  area 
of  a  leaf  on  a  stunted  one,  and  the  number  of  leaves  on  the 
larger  tree  will  be  many  times  the  number  on  the  smaller. 

In  the  summer,  when  the  soil  is  dry  and  the  air  is  hot, 
transpiration  may  cause  the  leaves  to  lose  water  so  rapidly 
that  they  droop,  and  we  say  that  the  plant  is  wilted.  Water 
has  passed  out  of  the  cells  of  the  leaf  faster  than  the  water- 
conducting  tissue  has  brought  in  water  to  replace  it,  and  the 
cells  are  no  longer  distended  and  firm.  They  are  like  a  foot- 
ball that  is  only  partly  inflated.  After  a  heavy  shower  the 
plants  quickly  recover,  because  the  water  available  in  the 


The  Water  Relations  of  Leaves 


53 


soil  has  been  increased  and  more  water  is  taken  into  the  plant. 
The  shower  has  also  covered  the  leaves  with  a  film  of  water 
and  made  the  air  moist  around  them,  and  this  reduces  the 
water  loss.  Under  these  conditions,  the  cells  of  the  plant 
quickly  become  turgid,  —  that  is,  become  fully  distended 
with  water,  —  and  the  leaves  recover  their  firmness.  The 
leaves  of  many  plants  like  lettuce,  pumpkin,  and  ragweed 
depend  for  their  firmness  almost  entirely  upon  the  turgidity 
of  the  leaf  cells. 

The  balance  between  the  rate  of  water  supply 
and  the  rate  of  water  loss  is  the  most  important 
water  relation  of  the  plant. 

The  water  balance  illustrated.  The  in- 
ternal water  balance  of  the  plant  may  be 
crudely  illustrated  by  a  glass  tube  with  an 
inverted  porous  porcelain  cup  sealed  to  one 
end  and  with  a  stopcock  attached  near  the 
other  end.  If  the  cup  and  tube  are  filled 
with  water  and  the  open  lower  end  of  the 
tube  is  placed  in  a  dish  of  mercury,  the 
mercury  will  rise  as  the  water  evaporates 
through  the  porous  surface  of  the  cup.  If  we 
nearly  close  the  bottom  of  the  tube  by  means 
of  the  stopcock,  the  rate  at  which  the  mer- 
cury rises  is  diminished.  This  is  because  the 
evaporation  is  decreased  as  the  amount  of 
water  supplied  to  the  cup  is  lessened.  If  we 
open  the  stopcock,  but  cover  the  outside  of 
the  cup  with  a  thin  layer  of  some  substance  ing  up  of  water  in 
like  wax,  which  does  not  allow  water  to  pass  a  ^ee  through  tran- 

\  t       spiration    from    the 

through  it  freely,  the  rate  of  evaporation  will  leaves. 


54 


Science  of  Plant  Life 


again  be  checked.  This  time  the  water  can  pass  through  the 
tube  freely,  but  it  cannot  evaporate  through  the  cup  so 
rapidly  because  of  the  wax  covering. 

How  plants  are  adjusted  to  maintain  the  water  balance. 
Plants  become  modified  in  many  ways  in  response  to  the  con- 
ditions of  water  supply  and  water  loss  under  which  they  grow. 
Among  the  adjustments  that  help  plants  to  maintain  an 
advantageous  water  balance  under  dry  conditions  are : 

(i)  Thickened  cuticle  and  "bloom."  The  cuticle  of  a  leaf 
checks  transpiration  as  does  the  wax  film  in  the  experiment, 
and  in  plants  of  dry  climates  the  cuticle  may  be  so  thick  as 
to  reduce  transpiration  through  the 
epidermis  to  almost  nothing.  There 
are  many  plants  which  secrete,  in 
addition  to  the  cuticle,  particles  of 
wax  on  their  leaves  or  other  parts. 
This  is  the  so-called  "  bloom  "  which 
may  be  seen  on  the  leaves  of  the 
houseleek  and  cabbage  and  on  the 
fruits  of  the  grape,  plum,  and  blue- 
berry. The  bloom  consists  of  a  layer 
of  wax  particles  scattered  thickly  over 
the  surface  of  a  leaf  or  fruit.  It  forms 
a  layer  that  is  nearly  impervious  to 
water  and  helps  to  reduce  water  loss 
E.s.ciements  through  the  epidermis. 
FIG.  35.  Vertical  sections  of  (2)  Compact  leaves.  A  plant  may 
leaves  of  Mertensia  showing  b  adjusted  to  an  inadequate 

differences  in  structure  when 

growing    in   moist,   shaded  water  supply  by  the  development  of 

situation   (above),  and  when    leayes  ^.^  compact   tlSSUCS.      In  SUCh 
growing     in     dry,     intensely 

lighted  situation  (below).         leaves  the  intercellular  spaces  are  much 


The  Water  Relations  of  Leaves  55 

reduced,  so  that  evaporation  from  the  mesophyll  cells  is 
greatly  lessened.  In  extreme  cases  the  mesophyll  cells  are 
all  of  the  compact  palisade  type,  which  leaves  the  minimum  of 
air  space  within  the  leaf. 

(3)  Small  leaf  area.  A  third  way  in  which  plants  become 
adjusted  to  dry  conditions  is  by  a  decrease  in  the  total  leaf 
area.  When  a  plant  is  brought  into  the  house  in  autumn, 
it  drops  a  number  of  leaves.  The  air  inside  most  houses 
being  much  drier  than  the  air  outside,  transpiration  is  greatly 
increased.  As  the  water  supply  remains  about  the  same, 
the  dropping  of  a  few  leaves  restores  the  internal  water  bal- 
ance of  the  plant.  Some  trees,  like  the  cottonwood,  shed 
part  of  their  leaves  during  a  summer  drought.  If  a  wet 
period  follows,  more  leaves  may  be  added,  and  in  this  way  a 
nearly  uniform  water  balance  is  maintained. 

That  plants  growing  under  moist  conditions  have  larger 
leaves  and  more  leaves  than  the  same  kinds  of  plants  growing 
under  dry  conditions  has  been  noted  by  every  one.  The 
contrast  may  be  observed  by  comparing  weeds  that  grow  along 
the  base  of  a  railroad  embankment  or  the  high  bank  of  a  stream 
with  those  that  grow  near  the  top. 

Transplanting  and  the  water  balance.  When  the  skillful 
gardener  transplants  a  tree,  he  cuts  off  a  number  of  branches 
to  reduce  the  number  of  leaves,  in  order  that  the  plant  may 
not  dry  out  before  new  water-absorbing  roots  are  developed. 
Before  lettuce,  tomato,  and  cabbages  are  lifted  for  trans- 
planting, the  plants  should  be  watered  and  allowed  to  become 
turgid ;  water  should  be  poured  into  the  holes  in  which  they 
are  placed,  before  the  soil  is  closed  in  around  the  plants.  It 
is  customary  also  to  cover  the  plants  with  boards  or  paper 
covers  so  as  to  reduce  the  transpiration.  Maintaining  the 


56  Science  of  Plant  Life 

water  balance  in  transplanted  plants  may  prevent  the  loss  of 
many  of  them  and  may  save  weeks  of  delay  in  the  maturing 
of  the  crop. 

The  water  balance  and  plant  habitats.  The  place  where 
a  plant  grows  naturally  is  called  its  habitat.  The  willow  grows 
beside  a  stream  and  the  cactus  grows  in  the  desert,  each  in  its 
natural  habitat.  If  we  put  the  willow  in  the  desert  and  the 
cactus  on  a  wet  stream  bank,  both  die.  This  means  that 
the  conditions  that  make  up  each  habitat  are  favorable  to 
one  kind  of  plant  and  not  to  another.  The  conditions  in- 
clude not  only  the  kind  of  soil  and  the  amount  of  soil  water, 
but  also  the  evaporative  power  of  the  air.  In  selecting 
plants  that  may  live  in  a  particular  habitat,  the  great  im- 
portance of  the  dryness  or  the  moistness  of  the  air  is  to  be  kept 
in  mind.  Plants  whose  leaves  are  soft  and  transpire  water 
rapidly  can  succeed  only  in  moist  air,  while  those  that  have  a 
low  transpiration  rate  can  maintain  a  suitable  water  balance 
only  in  a  dry  atmosphere.  This  is  one  of  the  reasons  why  on 
a  southern  slope  we  find  a  set  of  plants  that  are  different  from 
those  on  the  northern  slope. 

Recent  studies  have  shown  that  the 
leaves  of  plants  growing  near  the  bottom 
of  a  ravine  transpire  water   10  to  20 
times   as   fast  as   do  those  of   plants 
growing    higher    up    on    an    adjoining 
southern  slope.     Doubtless,  each  year 
E.s.ciements   seeds  of  plants  that  grow  in  the  low 
of  KTfSta   §«!  germinate  on  the  upper  part  of 
plant.     The   upper   figure   the   slope  ;    but  each  year  the  plants 


shows    an    aerial    leaf,    the    ^^  spring   from  those  seec}s  are  elimi- 
lower    figure    a    submerged 

leaf.  nated  through  their  inability  to  get  the 


The  Water  Relations  of  Leaves 


57 


water  needed  for  their 
high  rate  of  transpira- 
tion. There  are  plants 
like  the  dandelion  that 
can  adjust  themselves 
to  both  these  condi- 
tions. Most  plants, 
however,  cannot  do  this, 
and  those  with  a  high 
transpiration  rate  die 
off  on  a  dry  hillside, 
while  those  with  a  low 
transpiration  rate  sur- 
vive. This  indicates 
only  one  of  the  factors 
which  must  be  taken 
into  account  in  the  selec- 
tion Of  plants  for  par-  FlG"  37'  Submerged  plants.  From  left  to  right: 
.  eelgrass  (Vallisneria),  naiad  (Najas),  water  weed 

ticular    habitats  ;     Other     (Elodea),  and  pond  weed  (Potamogeton). 

factors     will     be    con- 
sidered in  connection  with  the  study  of  stems  and  roots. 

Submerged  and  floating  leaves.  An  examination  of  a 
submerged  leaf  of  any  pondweed  shows  that  it  has  no  stomatal 
openings.  The  floating  leaves  of  water  lilies  and  other  pond 
plants  have  stomata  only  on  the  upper  surfaces.  Evidently 
submerged  plants  have  no  transpiration.  It  is  also  certain 
that  they  get  their  carbon  dioxid  directly  from  the  water 
through  the  epidermis,  for  carbon  dioxid  is  found  dissolved  in 
pond  waters,  often  in  larger  proportion  than  in  the  air. 

In  water-lily  leaves  the  upper  surface  is  covered  by  a 
cuticle  that  is  not  readily  made  wet,  and  it  has  stomata  that 


Science  of  Plant  Life 


do  not  open  until  the 
leaf  is  above  water.  If 
the  leaves  are  raised 
entirely  above  the  sur- 
face of  the  water,  as 
sometimes  happens 
when  the  plants  are 
crowded,  both  surfaces 
develop  stomata. 

Desert  plants  and 
water  storage.  In  the 
desert,  where  the  air  is 
very  dry  and  the  scanty 
rainfall  is  confined  to 
one  or  two  periods  in 
the  year,  plants  have 
very  great  difficulty  in 
securing  water.  The 
perennial  plants  have 
various  ways  of  con- 
serving water  from  one 
rainy  period  to  the 
next.  The  barrel  cac- 
tus has  no  leaves  at 
all,  and  the  stem  is  a 
thick  cylinder  composed  largely  of  water-storage  tissue;  it 
may  live  without  water  for  2  years  or  longer.  Some  of  the 
desert  shrubs  have  leaves  during  the  rainy  periods  only,  and 
these  are  shed  as  soon  as  the  drought  comes.  Still  others, 
like  the  agaves,  have  very  thick,  leathery  leaves  with  much 
internal  water-storage  tissue  and  a  very  low  transpiration  rate. 


FIG.  38.  Desert  plants,  including  forms  of  Cereus, 
prickly  pear  (Opuntia),  Yucca,  Euphorbia,  and  cen- 
tury plant  (Agave). 


The  Water  Relations  of  Leaves 


59 


Adjustment  to  desert  conditions  by  ability  to  withstand 
drying.  Another  group  of  plants  is  adjusted  to  desert  condi- 
tions by  being  able  to  with- 
stand complete  drying.  The 
resurrection  plant  (Fig.  39) 
of  Texas  is  an  example  of 
this  group.  During  the 
rainy  season  it  is  green  and 
has  its  many  scale-leafed 
branches  spread  out  for  food 
manufacture  and  growth. 
When  drought  comes,  the 
plant  dries  out  completely 
and  its  branches  curl  upward 
until  it  is  in  the  form  of  a 
ball.  In  this  condition  it 
may  be  blown  about  by  the  FIG 
wind  and  remain  dormant 
for  weeks  and  months,  all 
of  its  physiological  processes 
having  ceased.  When  the  plant  again  becomes  wet  it  unfolds, 
and  its  processes  begin  anew.  In  the  eastern  United  States  we 
find  plants  of  this  same  type  in  the  lichens,  mosses,  and  small 
ferns  that  grow  on  the  bark  of  trees  and  on  bare,  dry  rocks. 

Plants  classified  according  to  their  water  relations.  In  the 
preceding  paragraphs  the  importance  of  the  water  require- 
ments of  plants  has  been  made  clear.  We  have  seen  that  the 
internal  water  balance  of  the  plant  is  of  great  importance  in 
modifying  its  physiological  processes  and  the  size  and  struc- 
ture of  its  organs.  Three  great  classes  of  plants  are  dis- 
tinguished on  the  basis  of  their  water  relations : 


Resurrection  plant  (Selaginella), 
in  growing  condition  (above),  and  the  same 
plant  in  a  dry  and  dormant  condition 
(below).  ' 


6o 


Science  of  Plant  Life 


(1)  The  plants  that  natu- 
rally live  where  the  evapora- 
tive power  of  the  air  is  intense 
and    the    available   water   is 
limited  are  called  xerophytes 
(Greek :  xeros,  dry,  and  phy- 
ton,    plant).     These   are   the 
plants  that  are  adjusted  to  a 
nearly  continuous  dearth   of 
water;     the     cacti,     agaves, 
yuccas,  and  sagebrush  of  our 
Western   plains    and    deserts 
are   striking    representatives. 
In  the  eastern  United  States 
there    are    less    marked    ex- 
amples of  xerophytes  in  the 
plants  that  live  on  dry  cliffs 

FIG.  40.   Evergreen  trees  growing  under    and  sand  beaches,  and  in  the 

extreme  xerophytic  conditions  in  Oregon.     mosses  and  HchenS  that  gTOW 
These     Pillars  of  Hercules"  are  by  the 

side  of  the  Lincoln  Highway.  on  trees  and  rocks. 

(2)  The    plants    that    live 

partly  or  wholly  submerged   in   the   water   are   known   as 
hydrophytes  (Greek  :  hudor,  water,  and  phyton,  plant) .     These 
plants  have  an  excessive  water  supply,  and  transpiration  is 
reduced  or  entirely  wanting.     In  this  class  are  included  the 
water  lilies,  pondweeds,  cat-tails,  bulrushes,  and  many  sedges. 
They  are  the  common  plants  of  fresh-water  ponds,  swamps, 
and  marshes  throughout  the  world. 

(3)  Between  these  extremes  are  the  mesophytes  (Greek: 
meso,  middle,  and  phyton,  plant),  by  far  the  largest  class  of 
plants.  They  have  a  medium  rate  of  transpiration  and  grow 


The  Water  Relations  of  Leaves  61 

best  with  a  moderate  water  supply.  In  this  group  are  in- 
cluded the  plants  that  yield  most  of  our  garden,  field,  and 
meadow  crops ;  also  most  of  the  forms  that  are  found  in  the 
maple,  beech,  and  elm  forests  of  the  Eastern  states,  and  in 
the  fir  and  spruce  forests  of  the  canons  and  bottom  lands  of 
the  Western  states. 

Xerophytes,  hydrophytes,  and  mesophytes  are  readily  dis- 
tinguished as  groups  because  of  their  great  differences  of 
habitat  and  appearance.  But  it  is  not  always  easy  to  de- 
cide whether  a  particular  plant  is  a  xerophyte,  hydrophyte, 
or  mesophyte,  because  we  find  all  gradations  of  form  among 
plants  of  the  three  classes.  Nevertheless,  these  terms  are 
useful  in  describing  the  water  relations  of  many  plants. 

PROBLEMS 

1.  How  do  plants  that  are  wilted  in  the  late  afternoon  of  a  hot  summer  day 
recover  their  firmness  during  the  night,  even  though  there  is  no  rain  ? 

2.  Where,  near  your  home,  do  mesophytes,  xerophytes,  and  hydrophytes  occur? 

3.  In  what  regions  of  the  United  States  are  mesophytes  most  common? 

4.  In  what  parts  of  the  United  States  are  xerophytes  abundant? 

5.  In  what  parts  of  the  United  States  are  hydrophytes  common  ?    What  states 
have  very  few  hydrophytes? 

6.  What  xerophytes  furnish  useful  products  to  man  and  animals? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Six 

1.  Collect   from   various   plants   leaves   showing   the   autumn 
colors.     Make  a  record  of  the  colors  found  in  different  kinds  of 
trees. 

2.  Look  for  different  colors  assumed  by  leaves  of  the  same  kind 
of  tree  when  growing  in  shade  and  in  bright  sunlight. 

3.  Look  for  the  red  color  in  leaves  that  have  been  partly  shaded. 
What  is  the  effect  of  the  shade? 

4.  Cut  longitudinal  sections  through  the  stem  and  base  of  a  leaf 
that  is  about  ready  to  fall,  and  note  the  abscission  layer. . 

5.  Examine   some  evergreens  and  determine   the  number   of 
years  they  hold  their  leaves.     Differences  in  color  of  bark  will  help 
to  distinguish  the  growths  of  different  years. 

6.  Make  a  chlorophyll  extract  by  placing  leaves  in  warm  alcohol. 
Dilute  the  alcohol  with  a  little  water,  and  add  enough  benzine  to 
double  the  volume  of  the  solution.     Then  shake  thoroughly,  and 
allow  to  stand  until  the  benzine  has  separated.     The  benzine  will 
contain  the  yellow  pigments,  while  the  chlorophyll  will  remain  in 
the  alcohol. 


62 


CHAPTER   SIX 

LEAF   COLORATION   AND   THE   FALL   OF  LEAVES 

IN  spring  and  summer  the  most  prominent  feature  of  the 
landscape  is  the  green  color  of  the  vegetation.  The  most 
striking  feature  in  autumn  is  the  varied  colors  of  the  foliage 
on  the  trees  and  shrubs.  In  the  northern  provinces  of  Canada 
most  of  the  trees  are  evergreen,  and  the  most  abun- 
dant deciduous  trees,  like  the  aspen,  birch,  and  tamarack, 
merely  turn  yellow.  But  in  our  Northern  states  the  vivid 
greens  of  the  sugar  maple,  white  oak,  gum,  and  sumac  dis- 
appear in  a  blaze  of  red  that  contrasts  strongly  with  the 
yellows  of  the  hickory,  linden,  and  poplar  and  with  the  dark 
greens  of  the  hemlock,  spruce,  and  pine.  Every  one  who  has 
seen  the  colors  of  autumn  woods  and  the  annual  falling  of  the 
leaves  must  have  wondered  what  processes  go  on  within  the 
leaves  to  bring  about  these  changes. 

The  pigments  in  green  leaves.  We  can  best  approach  the 
matter  of  autumn  colors  by  inquiring  into  the  composition  of 
the  pigments  that  give  the  color  to  the  leaves  of  deciduous 
trees  in  summer  and  to  the  leaves  of  evergreen  trees  through- 
out the  year.  The  most  abundant  of  these  pigments  is  chlo- 
rophyll (Greek :  Mows,  green,  and  phyll,  leaf),  which  is  bright 
green  in  color.  In  addition  to  chlorophyll,  two  other  pigments, 
one  yellow  and  one  orange,  are  found  in  a  green  leaf.  These 
three  pigments  may  exist  quite  independently  of  one  another.1 

1  The  coloring  matter  in  a  green  leaf  is  composed  of  about  66  per  cent  green 
pigment  (chlorophyll) ;  23  per  cent  yellow  pigment  (xanthophyll) ;  and  10  per 
cent  orange  pigment  (carotin,  so  named  because  of  its  abundance  in  the  carrot). 
The  green  pigment  is  not  a  simple  substance,  however,  but  a  mixture  of  two 
kinds  of  chlorophyll,  one  of  which  is  blue-green  and  the  other  yellow-green. 
The  depth  of  the  green  color  in  a  leaf  depends  in  part  on  the  proportions  in 
which  these  various  pigments  are  combined. 

63 


64  Science  of  Plant  Life 

In  the  chloroplasts  all  three  are  present  at  the  same  time, 
so  that  we  cannot  distinguish  them  under  the  microscope. 
As  the  three  are  soluble  in  alcohol,  the  presence  of  the 
yellow  and  orange  pigment  does  not  become  apparent  when 
the  coloring  matter  is  extracted  from  leaves  by  means  of 
alcohol.  The  chlorophyll  within  a  leaf  is  constantly  breaking 
down,  and  new  chlorophyll  is  formed  in  the  chloroplasts  to 
take  its  place. 

Conditions  affecting  the  development  of  the  pigments. 
Chlorophyll  is  produced  only  in  the  presence  of  light,  but  the 
yellow  and  orange  pigments  develop  in  the  dark  as  well  as  in 
the  light.  When  we  lay  a  board  on  grass  or  shut  out  the 
light  to  blanch  the  leaves  of  celery,  the  green  color  gives  way 
to  yellow  or  orange.  Likewise  seedlings  grown  in  the  dark 
and  the  inner  leaves  of  head  lettuce  show  a  yellow  but  not  a 
green  hue ;  and  when  the  light  is  cut  off  from  a  green  leaf, 
the  green  pigment  disappears,  leaving  the  yellow  pigments 
visible.1  These  facts  make  it  clear  that  the  yellow  pigments 
do  not  require  light  to  develop,  while  the  green  pigment  does. 

There  are  a  number  of  conditions  besides  absence  of  light 
that  result  in  the  partial  or  complete  disappearance  of  the 
green  pigment,  but  these  affect  various  plants  quite  differ- 
ently. Low  temperature,  drought,  and  injuries  and  diseases 
of  various  kinds  may  interfere  with  the  nutrition  of  the  leaf ; 
even  a  slight  decrease  in  light  may  do  so.  All  these  factors 
tend  to  affect  the  green  pigment  more  than  the  yellow  and 
orange.  Although  these  same  influences  —  low  temperature, 
drought,  reduced  light,  injuries,  and  diseases  —  may  be  ef- 
fective at  other  seasons,  they  become  generally  operative  in 
iate  summer  and  autumn.  Hence  it  is  at  this  time  of  the  year 

1  A  number  of  evergreens  are  exceptions  to  this  rule. 


Leaf  Coloration  and  the  Fall  of  Leaves  65 

that  the  green  pigment  disappears  from  the  leaves  of  most 
deciduous  plants  and  unmasks  the  yellow  pigments  in  the 
chloroplasts.  There  is  every  gradation  in  the  readiness  with 
which  the  green  pigment  disappears  from  the  leaves  of  differ- 
ent species  of  deciduous  trees,  from  the  cottonwood,  in  which 
the  leaves  become  yellow  during  a  midsummer  drought,  to 
the  peach,  in  which  they  may  still  be  vivid  green  when  shed. 
In  evergreens  the  chlorophyll  is  less  sensitive  and  external 
conditions  are  not  so  effective  in  causing  changes  in  the  color 
of  the  leaves. 

The  red  pigment.  The  red  colors  of  autumn  leaves  are  not 
due  to  changes  in  the  content  of  the  chloroplasts,  but  to  the 
formation  in  the  cell  sap  of  a  red  pigment  called  anthocyan. 
This  same  pigment  is  present  in  the  cells  of  many  young  leaves 
in  early  spring.  It  occurs  also  in  beets,  in  red  cabbage,  in  the 
petioles  and  veins  of  many  different  kinds  of  leaves,  in  the 
Coleus  and  other  foliage  plants,  and  in  many  flowers.  The 
presence  of  anthocyan  in  the  cell  sap  makes  the  whole  cell 
red,  and  any  or  all  of  the  cells  may  develop  the  pigment. 

The  development  of  the  most  brilliant  red  coloring  of  au- 
tumn is  commonly  ascribed  to  the  action  of  frost.  This  ex- 
planation is  probably  incorrect,  for  careful  observation  indi- 
cates that  the  color  is  most  intense  when  a  moderately  low 
temperature  is  accompanied  by  bright  sunshine.  In  warm, 
cloudy  autumns  the  colors  are  more  likely  to  be  dull,  with  the 
yellows  predominant.  That  sunlight  is  important  in  the 
development  of  the  red  pigment  may  be  shown  also  by  an 
examination  of  a  leaf  that  has  been  closely  shaded  by  another. 
The  pigment  stops  so  abruptly  where  the  shade  begins  that 
a  perfect  print  of  the  uppermost  leaf  results. 

The  red  colors  of  the  fruits  of  peaches,  apples,  and  pears 


66  Science  of  Plant  Life 

likewise  are  due  to  anthocyan.  Here  again  we  may  see  the 
effects  of  sunlight  on  the  intensity  of  color  by  comparing  fruits 
from  the  brightly  illuminated  top  of  the  tree  with  others  from 
the  shaded  under  parts.  The  apples  grown  in  the  Northwest- 
ern states  are  more  brilliant  in  color  than  the  same  varieties 
grown  in  the  Eastern  states,  and  this  higher  coloration  is 
probably  due  to  exposure  to  more  intense  light. 

Among  different  plants  there  is  much  variation  in  the 
amount  of  light  that  is  required  for  the  development  of  an- 
thocyan colors.  This  accounts  for  the  great  variation  in  the 
brilliancy  of  autumn  coloration  in  different  years.  One 
autumn  affords  light  conditions  which  promote  the  forma- 
tion of  anthocyan  in  only  a  few  trees  and  shrubs ;  another 
autumn  furnishes  conditions  so  favorable  that  many  plants 
become  brilliant. 

The  brown  colors.  In  some  trees  the  leaves  turn  brown 
immediately  after  the  loss  of  their  chlorophyll.  In  other 
trees  the  leaves  may  first  turn  yellow  or  red,  and  then  grad- 
ually assume  the  shades  of  brown.  These  brown  colors  re- 
sult from  chemical  and  physical  changes  in  the  substances 
within  the  leaf.  Just  what  the  processes  are  is  not  fully 
understood,  but  it  is  reasonably  certain  that  tannins  and 
tannic  acid  are  connected  with  the  making  of  the  brown  pig- 
ments. The  dead  bark  of  trees  also  turns  brown,  probably 
because  of  chemical  processes  within  it  similar  to  those  which 
take  place  in  the  leaves. 

White  leaves.  One  occasionally  finds  on  plants  leaves 
that  are  wholly  or  partly  white.  This  is  simply  the  natural 
color  of  living  plant  tissues  that  lack  chlorophyll  or  other 
pigments.  The  protoplasm,  cell  sap,  and  cell  walls  are 
transparent  and  colorless.  The  presence  of  air  spaces  among 


Leaf  Coloration  and  the  Fall  of  Leaves  67 

the  cells  makes  these  tissues  appear  white.     The  ice  crystals 

of  which  snow  is  composed  are  transparent,  but  the  numerous 

air  spaces  between  the  crystals  reflect 

the  light  and  cause  it  to  appear  white. 

Ice  likewise  becomes  white  when  it  is 

filled   with   minute   air   bubbles.      The 

white  colors  of  leaves  and  flowers  merely 

show    the   natural    appearance    of    the 

parts  in  the  absence  of  chlorophyll  and 

other  pigments. 

The   causes  of  leaf  fall.     There  are 
two  distinct  stages  in  the  process   by 
which  plants  drop  their  leaves :  (i)  the  FIG.  41.   Longitudinal  sec- 
formation  at  the  base  of  the  petiole  of  *?n  .of   ^ase  .of  ,petiole' 

r  showing  abscission  layer  at 

two  or  more  plates  of  thin-walled  cells,  beginning  of  leaf  fall, 
known  as  the  abscission  layer;  this  takes 
place  during  the  development  of  the  leaves  and  may  require 
weeks  or  months  for  completion ;   and  (2)  the  actual  separa- 
tion of  the  cells  of  the  abscission  layer,  which  is  brought 
about  by  the  softening  or  dissolving  of  the  middle  layer  of 
the  walls  of  the  abscission  cells.     This  stage  of  the  process 
may  take  place  within  a  few  hours,  or  at  most  within  a  few 
days. 

The  plant  is  protected  from  disease  and  water  loss  at  the 
scars  left  by  the  falling  leaves,  through  the  addition  of  woody 
and  corky  materials  to  the  cell  walls  beneath  the  abscission 
layer.  This  protective  layer  is  formed  in  some  kinds  of 
plants  before  the  leaf  drops,  in  other  plants  after  the  leaf  has 
fallen. 

Conditions  promoting  leaf  fall.  After  an  abscission  layer 
has  developed,  there  are  many  climatic  and  soil  conditions 


68 


Science  of  Plant  Life 


that  may  accelerate  the 


FIG.  42.  Shagbark  hickory  twig. 
A  is  the  bud  scales  of  the  terminal 
bud  of  the  previous  year,  B  sev- 
eral petioles  remaining  attached 
after  leaf  fall,  and  C  the  terminal 
bud  that  will  develop  the  follow- 
ing spring.  Drawn  from  a  speci- 
men collected  in  December. 


falling  of  the  leaves.  Among  these 
are  low  temperature,  reduced  light 
intensity,  and  any  disturbance  of  the 
water  relations  of  the  plant  which 
results  in  internal  drought.  Disease 
and  insect  injuries  to  the  blade  fre- 
quently bring  about  abscission. 

Leaves  contain  food  materials 
when  they  fall.  The  materials  used 
in  building  the  cell  walls  in  a  leaf 
are  lost  to  the  tree  when  the  leaf 
falls,  and  the  fallen  leaves  still  re- 
tain considerable  amounts  of  starch, 
sugar,  and  protein.  In  the  autumn, 
however,  photosynthesis  declines, 
and  the  amount  of  food  lost  by  a 
deciduous  tree  through  leaf  fall  is 
small  in  comparison  with  the  quan- 
tity that  has  accumulated  in  other 
parts  of  the  plant. 

Abscission  in  compound  leaves. 
In  many  compound  leaves,  like  the 
horse-chestnut,  ash,  and  hickory, 
abscission  first  takes  place  at  the 
base  of  each  leaflet.  Later  the 
petiole  is  cut  off  from  the  stem  in 
the  same  way.  Consequently  the 
leaflets  fall  first  and  the  petioles 
later.  In  the  king-nut  hickory  the 
petiole  remains  attached  to  the  tree 
through  the  following  year  (Fig.  42). 


Leaf  Coloration  and  the  Fall  of  Leaves 


Self-pruning.  A  large  number  of  our  common  trees,  like 
the  cottonwood,  maple,  and  elm,  develop  abscission  layers 
which  cut  off  twigs  and  some- 
times branches  an  inch  in  thick- 
ness. In  these  trees  we  have  twig 
fall  as  well  as  leaf  fall.  The 
falling  of  flowers,  and  of  fruits 
like  apples  and  nuts,  is  due  to 
the  abscission  layers  formed  in 
the  stems. 

Evergreen  and  deciduous  trees. 
In  the  Northern  states  many 
persons  have  come  to  think  that 
the  evergreen  habit  is  associated 
only  with  needle  leaves,  because 
in  the  North  the  evergreens  are 

FIG.    43.      Abscission    of    branches 

mostly  of  the  needle-leafed  type.  of  cottonwood.    Twigs  and  small 

But   in   the   Southern  States  there    branches  as  well  as  leaves  and  fruits 
,  ,  ,       -     ,   ,  , .,        are  cut  off  by  the  formation  of  abscis- 

are  many  broad-leafed  trees,  like  sion  layers 
the  magnolia,  rhododendron,  and 

holly,  that  are  also  evergreen.  Moreover,  the  tamaracks  of 
the  North  and  the  bald  cypress  of  the  South  furnish  examples 
of  needle-leafed  trees  that  are  deciduous.  If  we  include  the 
shrubs,  there  are  many  broad-leafed  plants,  both  in  the  North 
and  in  the  South,  that  have  the  evergreen  habit.  In  the  tropics 
most  of  the  trees  are  evergreen,  and  almost  all  have  broad  leaves. 
It  must  be  noted  that  even  in  the  case  of  evergreens  individual 
leaves  remain  on  the  trees  for  only  a  limited  number  of  years. 
The  leaves  of  the  evergreens  are  quite  different  structurally 
from  the  leaves  of  deciduous  trees.  The  evergreens  must  be 
able  to  withstand  freezing  and  thawing,  and  also  the  dry 


7o 


Science  of  Plant  Life 


Lee  Moorhouse 

FIG.  44.     Deciduous  trees  in  winter,  on 
Umatilla  Indian  Reservation,  Oregon. 


Caspar  W .  Hodgson 

FIG.  45.     Evergreen  trees  in  winter,  in 
Yosemite  Valley. 


Leaf  Coloration  and  the  Fall  of  Leaves  71 

winds  of  winter,  which  cause  transpiration  even  when  the 
ground  is  frozen.  This  suggests  that  their  usual  transpiration 
rate  must  be  very  low  in  comparison  with  that  of  the  decidu- 
ous trees. 

Evergreen  versus  deciduous  habit.  In  temperate  regions, 
where  there  are  great  changes  in  temperature  and  moisture, 
the  deciduous  and  the  evergreen  habit  each  has  certain  ad- 
vantages. The  advantages  of  the  evergreen  habit  are:  (i) 
that  the  leaves  can  manufacture  food  even  when  the  tem- 
perature is  low ;  (2)  that  with  their  low  water  requirement, 
evergreens  can  withstand  drier  conditions  throughout  the 
year ;  (3)  that  the  tree  does  not  waste  so  much  material  each 
year  in  the  construction  of  a  complete  set  of  new  leaves. 
The  disadvantages  of  the  evergreen  habit  are :  (i)  that  the 
heavy  cuticle  and  compact  tissues  which  aid  in  conserving 
water  interfere  with  rapid  photosynthesis ;  (2)  that  the  lower 
rate  of  food  manufacture  prevents  rapid  growth  ;  (3)  that  the 
leaves  lose  in  efficiency  by  their  longer  service  on  the  trees. 

The  advantages  of  the  deciduous  habit  are :  (i)  that  the 
leaves,  being  renewed  each  year,  are  more  efficient  organs  of 
food  manufacture ;  (2)  that  the  leaves,  with  less  cuticle  and 
with  tissues  less  compact,  are  better  fitted  for  rapid  food 
manufacture ;  (3)  that  the  total  leaf  area  may  be  much 
larger  than  in  the  case  of  the  evergreens ;  (4)  that  the  trees 
are  better  fitted  to  withstand  the  winter  drought,  because 
at  that  season  the  entire  tree  is  covered  with  cork.  The  dis- 
advantages of  the  deciduous  habit  are:  (i)  that  the  food- 
manufacturing  season  is  only  from  5  to  8  months,  as  com- 
pared with  from  8  to  10  months  in  the  evergreens ;  (2)  that 
each  year  a  large  amount  of  food  material  is  needed  to  make 
an  entirely  new  set  of  leaves. 


72  Science  of  Plant  Life 

Finally,  we  must  observe  that  there  are  in  trees  all  gradations 
between  the  deciduous  and  evergreen  habits.  In  the  rainy 
tropics  there  are  many  delicate-leafed  evergreens.  In  the  dry 
tropics  the  evergreens  have  thick,  fleshy  leaves,  or  they  may 
be  quite  leafless. '  Some  plants,  like  the  holly  and  the  Virginia 
creeper,  may  be  of  the  deciduous  habit  in  the  North  and  of  the 
evergreen  habit  in  the  South.  Some  deciduous  trees,  like  the 
cherry,  when  planted  within  the  tropics  become  evergreen; 
while  the  magnolia,  which  is  evergreen  in  the  Southern 
states,  becomes  deciduous  when  grown  in  a  colder  climate. 
Evidently  leaf  habits  are  adjustments  to  climatic  conditions, 
especially  to  conditions  of  temperature  and  moisture. 

PROBLEMS 

1.  What  are  the  commonest  evergreen  trees  and  shrubs  of  your  locality? 

2.  What  trees  of  your  vicinity  develop  leaves  earliest  in  the  spring?     What 
trees  put  out  their  leaves  last  ? 

3.  What  trees  develop  flowers  before  putting  out  their  leaves? 

4.  What  trees  drop  their  leaves  first  in  autumn  ?     What  trees  drop  their  leaves 
last  in  autumn? 

5.  What  trees  shed  part  of  their  leaves  during  the  summer? 

6.  During  how  many  months  does  each  of  these  trees  carry  on  food  manu- 
facture? 

7.  During  how  many  months  do  the  evergreen  trees  of  your  locality  manufac- 
ture food? 

8.  For  how  many  months  do  the  three  leading  crop  plants  of  your  locality  carry 
on  photosynthesis? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Seven 

1.  Make  a  thin  paste  by  boiling  a  little  starch  in  water.     Test 
a  small  portion  of  the  paste  with  iodin.     Test  for  sugar  with  Bene- 
dict's solution  l ;    then  add  diastase  solution  (malt  extract  pur- 
chased at  the  drug  store  may  be  used)  to  the  starch  paste ;  after 
it  has  stood  in  a  warm  place  for  some  hours,  test  it  again  for  starch 
and  .for  sugar.     What  is  meant  by  digestion  of  starch?     What 
enzyme  brings  this  about? 

2.  Test  for  starch  a  leaf  from  a  green  plant  that  has  been    in 
sunlight  by  placing  it  (i)  in  boiling  water  for  one  minute,  (2)  in 
hot  alcohol  until  bleached,  and  (3)  in  an  iodin  solution.     If  starch 
is  present,  put  the  plant  in  the  dark,  after  cutting  off  one  of  its 
leaves.     Place  the  severed  leaf  in  a  covered  dish  containing  moist 
filter  paper,  and  set  the  dish  in  the  dark.     On  the  following  day 
test  the  severed  leaf  for  starch ;   also  test  for  starch  a  leaf  freshly 
cut  from  the  plant  that  was  placed  in  the  dark.     What  do  the 
tests  show  regarding  the  movement  of  carbohydrate  out  of  leaves  ? 

3.  Test  seeds  of  corn,  bean,  and  almond  for  starch,  by  placing  a 
drop  of  iodin  solution  on  a  freshly  cut  section  of  each  seed. 

4.  Test  the  same  kinds  of  seeds  for  oil  by  scraping  the  seeds 
and  crushing  the  scrapings  on  a  sheet  of  white  paper.     Oil  makes 
the  paper  translucent.     Which  contains  the  most  oil  ? 

5.  Test  the  same  kinds  of  seeds  for  protein  by  placing  a  drop  of 
nitric  acid  on  a  section  of  each  seed.     Nitric  acid  stains  protein 
yellow. 

6.  If  time  permits,  other  organs  of  the  plant  may  be  tested 
for  food  in  the  same  way,  and  starch  grains  may  be  studied  under 
the  microscope.     Scrapings  from  potato,  the  root  of  the   canna, 
and  various  grains  will  furnish  material  for  the  study. 

1  Benedict's  solution:  Dissolve  173  gm.  copper  acetate  and  100  gm.  anhy- 
drous sodium  carbonate  in  600  cc.  of  distilled  water.  Filter,  and  add  water 
until  the  total  solution  is  850  cc.  Dissolve  17.3  gm.  of  copper  sulfate  in  100  cc. 
of  water  and  dilute  to  150  cc.  Then  mix  the  two  solutions,  which  together  will 
make  a  liter  of  Benedict's  solution.  It  is  ready  for  use  immediately  and  keeps 
indefinitely.  To  make  a  test  for  sugar,  add  3  cc.  of  Benedict's  solution  to  i  cc. 
of  the  solution  to  be  tested,  and  boil.  If  sugar  is  present,  a  red  or  yellow  pre- 
cipitate will  appear. 

73 


CHAPTER   SEVEN 

DIGESTION,   TRANSFER,   AND   ACCUMULATION   OF   FOODS 

WE  have  seen  that  starch  is  formed  in  the  leaves  of  a  plant 
when  it  is  exposed  to  light.  We  have  also  learned  by  ex- 
periment that  starch  disappears  from  leaves  at  night,  but 
that  if  a  leaf  is  removed  from  a  plant  it  will  still  contain  starch 
the  next  day.  Furthermore,  in  many  plants  like  the  potato, 
turnip,  or  corn,  we  find  starch  in  parts  of  the  plant  far  re- 
moved from  the  leaves.  These  facts  indicate  that  the  starch 
is  transferred  from  the  leaves  and  is  accumulated  in  the 
stems,  roots,  or  seeds.  In  the  present  chapter  we  shall  learn 
how  this  is  done. 

Digestion  of  starch.  Starch  is  insoluble  in  water.  It  does 
not  dissolve  in  the  cell  sap,  and  the  starch  within  the  cells  is 
not  divided  into  particles  small  enough  to  pass  through  the 
cell  walls,  before  it  can  be  moved  from  one  part  of  the  plant 
to  another,  or  even  from  one  cell  to  another,  it  must  be  changed 
into  some  substance  that  is  soluble.  The  process  of  changing 
starch  into  a  soluble  substance  has  been  carefully  studied ; 
and  we  know  that  starch  is  first  converted  into  maltose  and 
that  the  maltose  is  further  split  into  glucose  (page  28).  Glu- 
cose is  readily  soluble  in  water  and  consequently  can  be  passed 
from  cell  to  cell  and  so  transferred  to  any  part  of  the  plant. 
The  changing  of  insoluble  substances  like  starch  into  simpler 
soluble  substances  like  glucose  is  called  digestion.  Unlike 
animals,  plants  have  no  special  organs  of  digestion.  All 
their  living  cells  are  capable  of  digesting  the  insoluble  sub- 
stances that  are  required  for  their  nutrition. 

Digestion  brought  about  by  enzymes.  Digestion  is  brought 
about  by  substances  called  enzymes.  These  are  produced  by 
the  living  protoplasm  of  the  cells.  A  large  number  of  differ- 

74 


Digestion,  Transfer,  and  Accumulation  of  Foods     75 

ent  kinds  of  enzymes  have  been  recognized  in  plants ;  each 
enzyme  digests  only  one  particular  kind  of  food,  and  there 
must  be  a  different  enzyme  to  digest  each  kind  of  food  within 
the  cell.  The  enzyme  which  digests  starch  is  called  diastase. 
The  enzyme  that  digests  fats  is  called  lipase.  There  are  other 
enzymes  which  act  upon  the  insoluble  forms  of  protein  and 
render  them  soluble.  It  seems  probable  that  enzymes  are 
concerned  in  the  principal  activities  of  all  living  cells.  With- 
out them  there  could  be  none  of  the  rapid  changes  in  foods 
that  are  necessary  for  the  transfer  of  foods  within  the  plant 
and  for  carrying  on  the  other  processes  described  in  this  and 
the  next  chapter. 

It  is  interesting  to  know  that  if  an  enzyme  is  put  in  a  test 
tube,  with  the  appropriate  food  substance,  it  will  bring  about 
digestion  the  same  as  if  it  were  in  the  living  cell.  This  proves 
that  digestion  is  not  directly  carried  on  by  the  living  protoplasm, 
and  that  to  be  digested,  foods  do  not  need  to  be  in  contact 
with  living  matter.  It  requires  but  a  very  minute  quantity 
of  enzyme  to  digest  a  large  amount  of  the  particular  food  upon 
which  it  acts ;  for  example,  a  preparation  of  an  enzyme  ex- 
tracted from  the  pancreas  of  an  animal  was  found  to  digest 
2,000,000  times  its  weight  of  starch.  The  amount  of  diastase, 
therefore,  that  is  needed  in  a  mesophyll  cell  in  order  to  trans- 
form to  sugar  the  starch  in  that  particular  cell,  is  so  small  that 
it  cannot  be  measured. 

Accumulation  of  food.  A  healthy  plant  usually  manu- 
factures more  food  than  it  uses  immediately.  In  the  potato, 
surplus  food  is  carried  to  underground  stems,  the  tubers,  and 
is  there  stored.  Turnips  and  beets  are  examples  of  plants 
that  accumulate  excess  food  in  their  roots.  In  the  maple,  it 
accumulates  in  the  branches,  trunk,  and  roots.  In  the  cab- 


Science  of  Plant  Life 


U.  S.  Dept.  of  Agriculture 

FIG.  46.     An  orchard  in  California.     The  excess  food  of  the  tr. 
accumulated  in  the  fruits. 

bage,  food  is  stored  in  the  cluster  of  leaves  at  the  top  of  the 
stem.  In  corn  and  cereals,  the  excess  food  finally  accumu- 
lates in  the  grain.  In  the  century  plant,  a  considerable  part 
of  the  excess  food  is  stored  in  the  thick,  fleshy  leaves;  the 
process  of  accumulation  may  go  on  from  20  to  30  years,  and 
the  total  quantity  of  food  stored  may  amount  to  many  pounds. 
In  nature  such  accumulated  foods  are  used  for  another  season's 
growth  of  the  plant  or  in  starting  the  growth  of  its  offspring. 
Before  insoluble  foods  are  transferred  in  the  plant,  they  are 
digested  or  made  soluble  by  enzymes.  When  these  soluble 
substances  accumulate  in  the  cells  of  storage  organs,  they  fre- 
quently are  transformed  again  into  an  insoluble  form.  For 
example,  starch  formed  in  potato  leaves  is  transferred  through 
the  plant  to  the  underground  tubers  in  the  form  of  glucose, 


Digestion,  Transfer,  and  Accumulation  of  Foods     77 


U.  S.  Dept.  of  Agriculture  (E.  L.  Adams) 

FIG.  47.  A  field  of  shocked  rice  in  California.  The  surplus  food  of  the  rice  plant 
accumulates  in  the  seeds.  These  are  more  used  than  any  other  one  article  of  diet 
by  man. 

and  there  it  accumulates  in  the  cells  in  the  form  of  starch.  It 
is  believed  that  the  same  enzymes  which  change  the  starch  to 
glucose,  under  suitable  conditions  change  the  glucose  back 
again  to  starch,  and  that,  in  general,  the  enzymes  that  digest 
foods  are  the  agents  that  build  them  up  again  into  the  more 
complex  insoluble  forms. 

Kinds  of  food  accumulated.  In  any  given  plant  in  which 
food  is  accumulated,  protein,  carbohydrate,  and  fat  are  all 
present.  Depending  on  the  plant,  however,  the  amount  of 
any  one  of  these  may  be  very  great  or  it  may  be  so  small  as 
to  be  practically  negligible.  In  the  sugar  cane  and  sugar  beet 
the  excess  food  occurs  mainly  in  the  form  of  cane  sugar  (su- 
crose). In  the  potato  it  is  almost  wholly  starch.  The  grains 
of  wheat,  oats,  and  rice  contain  mostly  starch,  but  also  some 
protein.  In  sweet  corn  there  are  both  sugar  and  starch;  in 
field  corn  there  is  mostly  starch.  In  both  kinds  of  corn  there 
are  measurable  quantities  of  protein  and  oil.  In  the  soy  bean 


Science  of  Plant  Life 


Bruce  Fink 

FIG.  48.     A  sugar-cane  field  in  Porto  Rico.     The  excess  food  is  accumulated  in 
the  stems  in  the  form  of  sugar. 

and  peanut,  there  are  large  quantities  of  both  protein  and  oil. 
In  the  seeds  of  the  coconut,  flax,  and  cotton  there  is  a  large 
proportion  of  oil. 

Food-accumulating  plants  and  agriculture.  Throughout 
the  long  history  of  agriculture  the  principal  purposes  of 
agriculturists  have  been :  (i)  the  discovery  of  plants  that 
accumulate  large  amounts  of  food;  (2)  the  improvement 
of  these  plants  by  selection  of  seed  from  the  most  productive 
or  otherwise  most  desirable  individuals  ;  (3)  improvement  of 
the  methods  of  cultivation  to  provide  conditions  of  growth 
that  will  cause  the  largest  possible  amounts  of  foods  to  be 
accumulated  in  the  plants.  Sweet  corn  was  developed  through 
the  selection  of  seeds  with  a  view  to  securing  plants  having  a 


Digestion,  Transfer,  and  Accumulation  of  Foods    79 


large  sugar  content ;    field  corn  was  developed  through  the 
selection  of  seeds  for  starch   content.     The  original  sugar 


P2.2 


78.3 


W  84.6 


Ash,  1.4 


BANANA 


FIG.  49.  Diagrams  showing  percentages  of  water,  protein,  fat,  carbohydrate,  and  ash 
in  various  plant  organs.  Note  the  low  percentage  of  water  in  seeds  as  well  as  the 
similarity  of  content  of  the  potato  and  the  banana. 

beets  contained  less  thaji  5  per  cent  of  sugar ;  because  of 
careful  seed  selection  and  intelligent  fitting  of  the  environ- 
ment to  the  plants,  the  best  quality  of  beets  grown  today  con- 
tains as  high  as  22  per  cent.  The  original  potato  tubers 
weighed  about  an  ounce;  potatoes  weighing  several  pounds 
are  now  grown,  and  a  plant  may  produce  ten  times  as  many 
tubers  as  are  produced  by  the  original  plant. 

The  testing  of  wild  species  of  plants  for  possible  cultiva- 
tion and  use  as  sources  of  food,  and  the  introduction  of  species 
cultivated  in  other  countries,  is  being  carried  on  by  govern- 
ments in  all  parts  of  the  world.  Through  its  agricultural  ex- 
plorers the  United  States  has  probably  accomplished  more 


8o  Science  of  Plant  Life 

in  this  particular  field  than  have  all  other  countries  com- 
bined. Examples  of  plants  that  have  been  successfully  in- 
troduced into  this  country  through  the  efforts  of  government 
experts  are  the  drought-resistant  wheat,  dasheens,  figs,  dates, 
guavas,  kafir  corn,  milo,  and  new  varieties  of  oranges. 

PROBLEMS 

1.  In  the  recent  Yearbooks  of  the  Department  of  Agriculture,  look  up  the 
accounts  of  new  plant  introductions. 

2.  Why  is  it  important  that  the  United  States  should  grow  within  its  own 
boundaries  as  great  a  variety  as  possible  of  cereals,  fruits,  and  other  food- 
producing  plants? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Eight 

1.  Generate  some  carbon  dioxid  by  pouring  hydrochloric  acid 
on  marble  in  a  flask  with  a  long  delivery  tube.     Pass  some  of  the 
gas  through  clear    limewater   in  a  small   beaker.     What  is    the 
effect  ? 

2.  Blow  your  breath  through  a  tube  into  another  small  beaker 
containing  limewater.     Is  the  effect  the  same? 

3.  Arrange  each  of  two  small  flasks   with  a  two-hole  stopper 
and  two  tubes  as  shown  in  Figure  50.     In  the   first    flask   put 
flowers;  in  the  second  put   clear   limewater.      After    they    have 
stood  for  a  few  minutes,  draw  air  from  the  first  flask  through  the 
limewater  in  the  second.     Note  the  effect.      Does  the  respiration 
of  plants  produce  the  same  gas  as  the  respiration  of  animals  ? 

4.  Mark  young  leaves  of  fern,  corn  seedlings,  and  nasturtium 
as  suggested  in  the  text,  and  study  the  growth  of  the  three  kinds 
of  leaves. 

5.  Show  by  experiment  that  oxygen  is  necessary  for  the  growth 
of  seeds. 


3i 


CHAPTER   EIGHT 

THE   UTILIZATION    OF   FOODS 

IN  the  preceding  chapters  we  have  seen  how  food  is  manu- 
factured by  the  plant,  how  it  is  made  soluble  and  is  trans- 
ferred, and  how  the  excess  food  is  accumulated  in  various 
organs  of  the  plant.  Food  is  finally  utilized  by  the  cells  in 
respiration,  assimilation,  and  growth.  In  this  chapter  we  shall 
learn  the  meanings  of  these  terms  and  study  the  changes  that 
the  foods  undergo  in  connection  with  the  production  of 
energy,  the  making  of  protoplasm,  and  the  growth  of  the 
cells. 

Energy  necessary  to  plant  cells.  In  order  to  do  work, 
each  machine  in  a  manufacturing  establishment  must  be 
supplied  with  energy,  and  every  living  cell  in  a  plant  requires 
energy  for  carrying  on  its  work  of  repair,  growth,  and  move- 
ment. In  manufacturing  establishments  the  energy  is  usually 
generated  at  one  place  and  is  then  transmitted  by  means  of 
shafts  and  belts  or  wires  and  motors  to  all  parts  of  the  fac- 
tory. The  plant  cannot  transmit  energy  from  one  part 
to  another,  but  it  can  and  does  send  food  to  all  its  living  cells, 
and  from  this  food  each  cell  generates  within  itself  the  energy 
that  it  needs. 

Respiration.  A  steam  engine  is  supplied  with  energy  by 
the  oxidation  of  fuel  beneath  the  boiler  that  is  connected 
with  it.  A  cell  is  supplied  with  energy  by  the  oxidation  of 
food  within  it.  The  process  by  which  the  cells  obtain  energy 
through  the  oxidation  of  foods  is  called  respiration.  In  the 
process  oxygen  is  absorbed  and  carbon  dioxid  is  given  off. 
Respiration  takes  place  in  all  living  cells,  and  to  carry  on  this 
necessary  process  all  parts  of  the  plant  must  be  supplied  with 
oxygen.  The  leaves  and  stems  of  land  plants  obtain  their 

82 


The  Utilization  of  Foods 


oxygen  from  the  atmosphere,  and  the  roots  from  the  air  that 
is  in  the  soil.     Wet  soils  are  unsuited  to  the  growth  of  many 

plants,    not    because    of    the     w  — ^ 

water  present,  but  because  of 
the  lack  of  a  sufficient  oxygen 
supply  for  the  roots.  Drain- 
age is  a  valuable  agricultural 
practice  not  only  because  it 
removes  excess  water,  but 
also  because  it  draws  air 
(oxygen)  into  the  soil.  When 
the  farmer  breaks  the  crust 

on  the  surface,  he  is  making  it  possible  for  more  oxygen  to 
reach  the  roots  of  his  crop. 

The  plant  and  the  process  of  respiration  may  be  compared  to 
a  manufacturing  establishment  and  the  work  that  goes  on  in  it. 

The  power  stations   are  every  living  cell  of  root,  stem,  and 

i__r 


FIG.  50.     Carbon  dioxid  is  given  off  from 
the  respiration  of  the  flowers. 


The  machinery 
The  fuel 
The  process 
The  product 
The  waste 


leaf. 

is  the  protoplasm, 
is  foods,  especially  carbohydrates, 
is  the  combining  of  food  and  oxygen, 
is  energy, 
is  carbon  dioxid  and  water. 


The  working  hours    are  twenty-four  a  day. 

Respiration  and  photosynthesis  contrasted.  In  photo- 
synthesis, carbon  dioxid  and  water  are  combined  to  form  the 
complex  molecules  of  carbohydrates,  and  a  large  number  of 
atoms  of  oxygen  are  set  free  in  the  process.  When,  in  res- 
piration, the  complex  carbohydrate  molecules  are  again 
combined  with  oxygen,  simple  molecules  of  carbon  dioxid 


84  Science  of  Plant  Life 

and  water  are  formed.  In  photosynthesis,  the  energy  of  the 
sunlight  is  used  in  building  up  the  carbohydrates.  The 
energy  is  stored  in  the  carbohydrates,  and  it  may  be  released 
by  changing  them  back  to  the  simple  substances  out  of  which 
they  were  made.  When  we  wind  up  a  clock  spring,  we  put 
energy  into  the  tightened  coil.  When  the  spring  is  allowed 
to  uncoil,  this  energy  is  released  and  turns  the  wheels  of  the 
clock.  So  in  photosynthesis  the  energy  is  stored  in  the  car- 
bohydrates, and  this  energy  is  released  in  the  process  of  res- 
piration and  used  in  the  life  processes  of  the  cell. 

In  photosynthesis  In  respiration 

Oxygen  is  released.  Oxygen  is  consumed. 

Energy  is  accumulated.  Energy  is  released. 

Simple  molecules  are  built  up  Complex  molecules  are  broken 

into  complex  ones.  down  into  simple  ones. 

Plants  accumulate  food  and  Plants  consume  food  and  de- 
increase  in  weight.  crease  in  weight. 

Comparative  rates  of  respiration.  The  rate  of  respiration 
is  greatest  where  there  is  rapid  growth,  as  in  germinating 
seeds,  opening  flowers,  and  ripening  fruits.  In  some  of  these 
it  is  much  more  rapid,  bulk  for  bulk,  than  in  animals.  The 
lowest  rates  of  respiration  occur  in  seeds  and  other  dormant 
structures;  and  there  is  comparatively  little  respiration  in 
woody  stems  and  other  hard  parts  in  which  there  are  only  a 
few  living  cells. 

Respiration  and  the  shipping  of  fruits  and  vegetables.  How 
important  the  recognition  of  the  respiratory  requirement  of 
living  cells  is,  may  be  illustrated  by  the  difficulties  that  have 
been  met  with  in  shipping  fruits  and  bulbs.  Peaches,  during 
shipment,  sometimes  develop  brownish  spots  where  they 


The  Utilization  of  Foods  85 

touch  each  other.  These  spots  were  formerly  thought  to  be 
due  to  jarring  in  transportation,  but  they  are  now  known  to 
be  caused  by  packing  the  peaches  so  closely  that  the  air  does 
not  have  full  access  to  all  the  fruit.  The  respiration  of  the 
cells  at  the  points  of  contact  is  in  consequence  interfered 
with,  and  these  cells  are  suffocated  and  gradually  die.  Ships 
with  specially  ventilated  holds  are  used  in  importing  bulbs 
from  Holland  and  fruits  from  the  tropics.  The  building  of 
ventilated  holds  came  as  a  result  of  the  death  of  several  men 
who  attempted  to  unload  a  cargo  of  bulbs  from  an  unventilated 
bottom. 

Assimilation.  Another  part  of  the  food  constructed  by  the 
plant  is  used  for  repair  and  for  building  additional  protoplasm. 
A  machine,  if  allowed  to  stand  idle  and  uncared  for,  gradually 
goes  to  pieces.  The  cell  is  a  very  delicate  mechanism,  built 
of  highly  complex  substances,  and  the  protoplasm  of  the  ac- 
tive cell  requires  constant  repair.  The  foods  that  most  nearly 
approach  protoplasm  in  chemical  composition  are  the  pro- 
teins. Naturally  these  are  the  foods  most  readily  changed 
into  protoplasm  and  are  the  ones  mainly  used  in  the  process 
of  assimilation.  Assimilation  may  be  defined  as  the  process 
through  which  the  living  protoplasm  is  repaired  or  new  proto- 
plasm built  up  by  the  use  of  foods.  Assimilation  takes  place 
in  all  living  cells. 

Growth.  The  enlargement  of  plants  or  the  development  of 
new  structures  is  called  growth.  The  fact  about  plant  life  that 
is  most  familiar  to  all  is  that  when  a  live  seed  is  planted  in 
the  soil  it  germinates,  and  that  from  it  there  develops  a  seed- 
ling which  continues  to  enlarge  for  a  longer  or  shorter  time, 
depending  on  the  plant  and  the  conditions  of  growth.  The 
period  of  growth  may  be  a  month,  as  in  the  radish  in  mid- 


86  Science  of  Plant  Life 

summer,  or  it  may  be  hundreds  of  years,  as  in  some  trees.  In 
the  process  of  growth  vast  quantities  of  food  are  consumed. 
During  the  early  stages  of  a  plant's  development  most  of  the 
food  it  manufactures  is  used  in  this  way.  In  order  to  grow, 
a  plant  must  make  new  protoplasm,  develop  new  cell  walls, 
and  thicken  and  strengthen  old  cell  walls.  Growth  requires 
not  only  food,  but  energy  as  well.  Indeed,  a  considerable 
part  of  the  energy  derived  from  respiration  is  used  in  growth. 
We  might  expect  assimilation  and  food  consumption  to  be 
most  active  in  young  growing  parts,  and  that  this  is  the  case 
has  many  times  been  verified  by  experiment.  Growth  takes 
place  through  the  enlargement  of  cells  already  present  in  the 
plant,  through  cell  division,  and  through  modification  of  cells 
without  enlargement. 

The  making  of  the  cell  walls.  The  wall  that  surrounds  each 
cell  of  the  plant  is  composed  largely  of  a  substance  called 
cellulose,  which  is  secreted  by  the  living  protoplasm.  When 
the  cell  is  growing,  its  wall  is  exceedingly  thin  and  it  stretches 
as  the  cell  enlarges.  When  a  cell  divides,  a  new  wall  is  formed 
between  the  two  parts.  As  the  cell  grows  older,  new  layers 
of  cellulose  and  allied  substances  are  added.  In  some  tis- 
sues, as  in  the  shells  of  nuts,  the  walls  become  so  thick  as  to 
occupy  most  of  the  volume  of  the  cell.  In  other  tissues,  like 
the  mesophyll  of  leaves,  the  cell  walls  always  remain  thin. 

Chemically,  cellulose  is  a  carbohydrate,  closely  related  to 
sugar  and  starch.  Sugar  and  starch  are  the  plant  foods  that 
are  mainly  used  in  its  manufacture,  just  as  the  proteins  are 
mainly  used  in  the  building  of  new  protoplasm.  Cotton 
fiber  is  pure  cellulose,  and  exemplifies  the  strength,  lack 
of  color,  and  insolubility  in  water  that  are  characteristic  of 
cellulose. 


The  Utilization  of  Foods  87 

Conditions  for  growth.     The  conditions  most  favorable  for 
growth  are  abundant  water  supply  and  warm  temperatures, 


FIG.  51.     Results  of  an  experiment  to  show  effects  of  light  and  moisture  on  the  growth 
of  potato  shoots :  A ,  light  but  no  water ;   B,  light  and  water ;   C,  water  but  no  light. 

such  as  normally  occur  in  summer.  For  the  growth  of  the 
plant  as  a  whole,  strong  light  is  favorable  because  it  increases 
the  supply  of  food.  For  the  growth  of  leaves  in  particular, 
medium  light  is  generally  most  favorable.  In  darkness  the 
blades  of  many  plants  do  not  expand,  and  in  very  intense 
light  they  do  not  expand  fully  because  of  excessive  water 
loss. 

The  growing  regions  of  leaves.  By  watching  the  develop- 
ment of  leaves  on  any  common  herb,  or  on  the  trees  in  spring, 
we  can  see  that  growth  takes  place  rapidly ;  also,  that  growth 
ceases  when  the  leaves  have  developed  to  a  certain  rather 
definite  size.  After  the  leaf  is  mature,  further  enlargement 
will  not  take  place,  no  matter  how  favorable  to  growth  the 
external  conditions  may  be.  The  question  arises,  do  all  parts 
of  the  leaf  enlarge  equally,  or  do  some  parts  grow  more  than 
others  ?  There  is  one  characteristic  of  growing  tissue  that  will 


88 


Science  of  Plant  Life 


A 


help  us  in  answering  this  inquiry  :  young  tissue  is  very  tender, 
and  easily  broken,  while  old  tissue  is  stronger  and  firmer. 

Fern  leaves  grow  at  the 
apex.  The  fern  leaf  is  one 
that  may  be  studied  in  this 
connection,  for  the  growing 
portion  is  not  only  tender 
but  coiled  up,  and  its  unfold- 
ing may  be  noted  from  day 
to  day  by  marking  with  India 
ink  the  successive  positions 
of  the  coil.  In  the  Boston 
fern,  which  is  so  commonly  cul- 
tivated as  a  window  plant,  the 
leaf  may  continue  to  unfold 
for  weeks,  if  the  water  supply 
is  adequate  and  other  condi- 
tions are  favorable .  Evidently 
FIG.  52.  Growing  regions  (shaded  por-  m  the  ferns  the  growing  region 

tions)  of  leaves :  A .  leaf  of  fern ;   B.  grass    .  •,  1,111 

leaf;  and  C,  leaf  of  sunflower.  1S   at  the  aPCX  and   the   °ldest 

part  of  the  leaf  is  the  base. 

Growth  in  the  leaves  of  seed  plants.  The  flowering  plants 
have  either  parallel- veined  leaves  or  net-veined  leaves,  and 
the  place  of  growth  in  these  two  types  of  leaves  is  different. 
In  parallel- veined  leaves,  like  those  of  the  members  of  the 
grass  family,  the  growth  is  at  the  base.  What  boy  or  girl, 
walking  through  a  field  of  timothy  or  wheat,  has  not  pulled 
the  leaves  from  the  stems?  The  leaves  always  break  near 
the  base.  If  you  have  tasted  the  broken  end,  you  will  recall 
that  it  was  sweet  and  tender.  The  breaking  near  the  base, 
and  the  sweetness  there,  indicate  that  the  growing  region  of 


The  Utilization  of  Foods 




U.  S.  Dept.  of  Agriculture 
FIG.  53.     A  tobacco  field  in  Connecticut.     The  plants  are  grown  for  their  leaves. 

the  grass  leaf  is  at  the  base.  A  more  exact  determination  of 
the  growing  region  may  be  made  by  marking  a  young  grass 
leaf  into  equal  spaces  with  India  ink.  This  will  show  that  as 
the  leaf  develops,  it  is  continually  pushed  upward  and  outward 
from  the  node  where  it  is  attached.  This  mode  of  growth  is 
characteristic  not  only  of  grasses  but  of  many  other  plants 
having  parallel- veined  leaves. 

Net-veined  leaves.  The  net- veined  leaves  develop  differ- 
ently from  the  leaves  of  either  ferns  or  grasses.  An  exami- 
nation of  a  growing  net- veined  leaf  will  show  that  all  parts  are 
equally  firm.  The  best  method  of  study  is  to  mark  the  young 
leaf  into  equal  squares  by  means  of  two  series  of  parallel  lines 
at  right  angles  to  each  other.  A  geranium  or  nasturtium  leaf 
serves  well  for  this  purpose.  After  several  days  it  will  be 
seen  that  the  only  change  has  been  an  increase  in  size  of  the 


9o 


Science  of  Plant  Life 


V.  S.  Dept.  of  Agriculture 

FIG.  54.     Cutting  leaves  from  the  sisal  (a  form  of  century  plant)  for  making  the 
fiber  used  in  the  manufacture  of  binder  twine  in  Yucatan. 


FIG.  55.    Drying  sisal  fiber  in  Yucatan. 


U.  S.  Dept.  of  Agriculture 


The  Utilization  of  Foods 


squares.  The  lines  in  each 
direction  are  still  parallel. 
This  indicates  that  all 
parts  of  the  blade  are 
growing  equally. 

These  facts  regarding 
the  growth  of  leaves  may 
be  summarized  in  a  some- 
what different  way.  In  the 
ferns  the  last  part  of  the 
leaf  to  mature  is  the  apex. 
In  parallel-veined  leaves  a 
region  near  the  base  is  still 
in  a  growing  condition 
after  the  other  parts  are 
mature.  In  net- veined 
leaves  all  parts  of  the  blade 
mature  at  the  same  time. 

Leaves  as  sources  of  commercial  products.  Many  plants 
are  grown  or  collected  for  the  sake  of  their  leaves.  The 
most  nourishing  part  of  forage  crops  like  hay  and  alfalfa  is 
the  leaves.  Lettuce,  celery,  and  Swiss  chard  are  important 
garden  leaf  crops.  The  leaves  of  the  tobacco  plant  furnish 
the  basis  of  a  world- wide  industry.  The  conductive  bundles 
from  .the  leaves  of  certain  Mexican  agaves  furnish  the  sisal 
fiber  used  in  the  manufacture  of  binder  twine.  Manila  fiber 
is  made  from  the  bundles  of  the  leaves  of  a  Philippine  banana. 
The  leaves  of  the  tropical  palm,  raffia,  are  used  by  gardeners 
for  tying  up  plants,  and  by  others  for  the  making  of  orna- 
mental baskets.  Other  tropical  palms  furnish  the  fibers 
from  which  Panama  hats  are  woven.  The  eelgrass,  which 


Bruce  Fink 

FIG.  56.  Hat  palm,  Porto  Rico.  The  leaves 
are  split  into  strips  and  are  used  in  weaving 
hats. 


92  Science  of  Plant  Life 

grows  in  shallow  water  along  our  coasts,  has  been  found  to 
be  one  of  the  best  materials  for  packing  the  sound-proof, 
fire-proof,  and  heat-proof  walls  of  apartment  houses,  factories, 
and  cold-storage  warehouses.  Cocain,  caff  em,  digitalis,  the 
oils  of  mint  and  wintergreen,  and  other  substances  used  in 
medicine  are  derived  from  the  leaves  of  plants. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Nine 

1.  Examine  the  stem  systems  of  an  herb,  a  shrub,  and  a   tree 
and  make  sketches  of  them,  preferably  in  the  field. 

2.  Field  trip  to  study  herbs,  shrubs,  and  trees,  noting  the  great 
variety  of  forms  in  each  group.     A  visit  to  a  greenhouse  will  permit 
the  study  of  a  variety  of  tropical  species  and  of  methods  of  prop- 
agating plants. 

3.  Field  trip  for  the  study  of  annuals,  biennials,  and  perennials. 
Old  fields,  gardens,  roadsides,  and  woods  will  show  a  variety  of 
forms.     In  the  case  of  perennials  the  underground  parts  are   as 
interesting  and  important  as  the  above-ground  shoots.     How  does 
each  of  these  types  of  plants  pass  the  winter? 


93 


CHAPTER  NINE 

HERBS,  SHRUBS,  AND  TREES 

EVERY  one  who  has  occasion  to  grow  plants  needs  to  know 
something  about  the  length  of  life  of  the  plants  he  is  concerned 
with,  and  he  must  know  also  whether  they  have  herbaceous 
or  woody  stems.  For  example,  suppose  a  farmer  wishes  to 
determine  whether  it  will  be  more  profitable  to  grow  sweet 
clover  or  alfalfa  in  a  certain  field.  Before  planting  either  of 
these  crops,  he  should  know  that  one  of  them  is  a  biennial  and 
the  other  a  perennial,  because  all  his  plans  for  handling  the 
crop  will  depend  on  this  information.  Or  suppose  that  an- 
other man  wishes  to  have  a  permanent  border  of  flowering 
plants  about  his  lawn  to  obstruct  the  view  of  some  unattrac- 
tive fields  or  buildings.  He  can  choose  wisely  from  among 
the  hundreds  of  plants  listed  in  nursery  catalogs  only  when  he 
has  definite  information  about  the  longevity  of  the  plants 
and  as  to  whether  they  are  herbs,  shrubs,  or  trees.  A  clear 
understanding  of  the  classification  of  plants  on  the  basis  of 
their  length  of  life,  their  woodiness,  and  their  tendency  to 
form  single  large  trunks  or  a  number  of  stems  is  helpful  also 
in  any  study  of  the  structure  and  processes  of  stems. 

Longevity  of  plants.  Plants  differ  greatly  in  their  length 
of  life.  To  indicate  the  length  of  the  natural  life  periods,  the 
terms  annual,  biennial,  and  perennial  are  commonly  applied 
to  plants.  It  is  important  that  these  terms  be  clearly  under- 
stood before  the  subject  of  the  tissues  and  their  arrangements 
in  stems  is  taken  up,  because  in  each  of  these  length-of-life 
classes  certain  characteristics  are  associated  with  the  form 
and  development  of  the  stem. 

Annuals.  Most  of  our  common  garden  vegetables  and 
field  crops  are  started  from  seeds  in  early  spring.  The  seeds 

94 


Herbs,  Shrubs,  and  Trees 


95 


germinate ;  roots  and  shoots  develop  ;   and  by  midsummer  or 
autumn,  flowers  and  fruits  are  produced  and  new  seeds,  which 
contain  the  beginning  of  another  generation 
of  plants,  are  formed.     Then  the  plants  die. 
The  period  from  seed  germination  to  seed  pro-  ^ 

duction  is  called  the  life  period.  If  it  is  com-  Jj|^ 
pleted  within  a  single  growing  season,  the  plant 
is  called  an  annual  (Latin  :  annus,  year) .  Corn, 
lettuce,  radishes,  beans,  pumpkins,  morning- 
glories,  and  ragweeds  are  familiar  annual  plants. 
Biennials.  During  the  first  season  some 
plants  develop  only  leaves  and  roots  and  a  very 
short  stem.  The  root  is  usually  large  and  accu- 
mulates a  considerable  amount  of  food.  In  the 
second  season  growth  is  re-  . 

newed,  and  there  is  developed 
an  upright  stem  with  leaves, 
flowers,  fruits,  and  seeds. 
These  plants  which  pass  a 
winter  season  during  their 
vegetative  development,  and 
whose  life  period  includes  two 
different  growing  seasons,  are 
called  biennials  (Latin :  bien- 
nium,  space  of  two  years).  The  seeds  of  some  common 
weeds,  like  the  shepherd's  purse,  evening  primrose,  and  wild 
lettuce,  germinate  in  August  or  September,  and  a  little  rosette 
of  leaves  is  formed  close  to  the  ground.  Food  accumulates  in 
the  root  until  winter  comes.  The  following  spring  the  plants 
make  rapid  growth,  and  by  midsummer  they  have  blossomed, 
produced  seed,  and  died.  In  spite  of  the  fact  that  their  whole 


FIG.  57.  Moth  mullein,  a  biennial:  first 
season  rosettes  (in  foreground)  and 
mature  plant. 


96  Science  of  Plant  Life 

life  is  passed  within  a  twelvemonth  period,  these  plants  are 
called  biennials,  because  their  life  period  covers  parts  of  two 
vegetative  seasons. 

The  term  annual  or  biennial  as  applied  to  plants,  therefore, 
does  not  imply  any  definite  length  of  life  in  months.  Wheat 
may  be  grown  either  as  an  annual  or  as  a  biennial,  depending 
upon  whether  it  is  planted  in  the  spring  or  in  the  fall.1  Shep- 
herd's purse  and  wild  lettuce  not  infrequently  live  as  annuals 
in  nature.  The  commonest  biennials  of  the  garden  are  beets, 
carrots,  parsnips,  turnips,  and  cabbage.  In  the  first  four, 
large  amounts  of  food  are  accumulated  in  the  roots;  in  the 
cabbage  the  food  is  stored  in  the  enormous  terminal  bud,  the 
"  head."  These  stores  of  food  are  used  in  the  production  of 
seeds  the  following  year. 

Usually  biennials  and  annuals  are  herbs.  Biennials,  like 
annuals,  are  comparatively  small  in  size,  and  die  after  flowers 
and  seeds  have  been  produced. 

Perennials.  Perennials  (Latin :  perennis,  lasting  through 
the  year)  are  plants  that  live  for  a  number  of  years.  Some  of 
them,  as  for  example  certain  grasses,  produce  seed  during  the 
first  and  succeeding  years.  Other  perennials,  like  alfalfa,  form 
seed  at  the  end  of  the  second  and  succeeding  seasons.  Trees 
and  shrubs  usually  require  several  seasons'  growth  before 
seeds  are  produced.  The  century  plant  of  our  southwestern 
deserts  develops  vegetatively  for  25  or  30  years  before  it  pro- 
duces a  flowering  stem  and  seeds.  Then  it  behaves  like  an 
annual  or  a  biennial,  for  as  soon  as  the  seeds  are  mature  the 
whole  plant  dies.  This  calls  our  attention  to  the  interesting 

1  When  wheat  is  planted  in  the  spring  and  grown  as  an  annual,  it  is  called 
"  spring  wheat."  When  it  is  planted  in  the  fall  and  grown  as  a  biennial,  it  is 
called  "  winter  wheat."  The  same  variety  may  be  grown  either  as  spring  01 
winter  wheat,  but  in  practice  different  varieties  are  grown. 


Herbs,  Shrubs,  and  Trees 


97 


fact  that  in  annuals,  biennials,  and  a  few  perennials  there  is 
no  well-marked  period  of  senility  or  old  age.  They  die  sud- 
denly at  maturity,  immediately  after 
their  period  of  greatest  vigor.  Trees 
and  shrubs,  on  the  contrary,  have  a 
distinct  period  of  old  age  in  which 
the  physiological  processes  are  slowed 
down  gradually  until  the  plants  suc- 
cumb to  diseases  and  unfavorable 
conditions  which  they  could  have 
withstood  in  youth. 

Perennials  classified  according  to 
the  persistent  parts.  All  perennials 
add  new  leaves,  new  stems,  and  new 
roots  each  year;  but  they  may  be 
classified  roughly  according  to  the 
parts  that  persist  from  one  season  to 
the  next. 

Evergreen  trees  and  shrubs  are 
perennial  in  all  parts  of  the  plant  FIG.  58.  Century  plant  (Agave), 

-,  T-^      .  ,  JIT         showing  rosette  of  fleshy  leaves 

body.  Deciduous  tees  and  shrubs  and  flowering  stem  It  is  a 
are  perennial  in  their  stems  and  roots,  perennial,  but,  like  an  annual 
Many  herbaceous  perennials,  like  the  °r  a  bienni^'  .ll  dies  w  en  ll 

•*  flowers  and  fruits. 

cat- tails,    swamp    mallows,    peonies, 

trilliums,  and  bananas,  have  annual  above-ground  stems  but 
perennial  underground  stems  and  roots.  Dahlias  and  sweet 
potatoes  have  perennial  roots.  Potatoes  and  the  Jerusalem 
artichoke  (a  kind  of  sunflower)  have  perennial  thickened 
underground  stems  (tubers).  Tulips  and  hyacinths  have 
perennial  underground  stems  (bulbs).  These  examples  show 
that  perennial  plants  have  many  different  ways  of  bridging 


98 


Science  of  Plant  Life 


FIG.  59.  Group  of  California  Big  Trees  (Se- 
quoia gigantea).  An  idea  of  their  size  and 
height  can  be  gained  by  comparing  them 
with  the  man  standing  at  the  left  of  the  picture. 


over  unfavorable  seasons 
like  periods  of  cold  or 
drought. 

There  seems  to  be  no 
limit  to  the  length  of  life 
of  some  perennial  herbs, 
like  ferns,  the  May  apple, 
Solomon's  seal,  and  cer- 
tain grasses  and  mints. 
The  older  parts  die  each 
year,  and  new  parts  form 
at  the  other  ends  of  the 
underground  stems.  The 
plants  change  their  loca- 
tions slightly  each  year, 
one  end  of  the  stem  grow- 
ing forward  and  the  other 
end  dying  away.  There 
is  no  apparent  reason  why 
such  plants  should  not 
live  indefinitely,  perhaps 
longer  than  the  oldest 
trees;  but  no  one  part  of 
the  plant  lives  for  a  long 
time. 


Herbs,  shrubs,  and  trees.  Shrubs  and  trees  have  woody 
stems.  The  stems  of  herbs  lack  woody  tissues.  Our  garden 
and  field  crops  are  all  herbaceous  plants.  Their  stems  con- 
tain no  woody  tissue  and,  for  this  reason,  in  temperate  climates 
the  above-ground  parts  live  but  a  single  growing  season. 

The  principal  difference  between  shrubs  and  trees  lies  in 


Herbs,  Shrubs,  and  Trees 


99 


the  fact  that  shrubs  develop  numerous  slender  above-ground 
stems  from  a  single  base,  while  trees  develop  a  single  stem  or 
trunk.  This  distinction  may  be 
expressed  in  another  way  by 
saying  that  shrubs  branch  un- 
derground, while  trees  branch 
only  above  ground.  Most 
shrubs  are  less  than  10  feet  in 
height,  but  some,  like  the  stag- 
horn  sumac,  may  reach  a  height 
of  20  feet.  Most  trees  are  be- 
tween 25  and  200  feet  in  height, 
but  the  eucalyptus  tree  of  Aus- 
tralia and  the  giant  sequoias  of 
California  are  over  300  feet  in 
height,  and  the  massive  trunks 
of  the  latter  may  be  more  than  FlG.  6o.  Japanese  dwarf  pine.  Some 

30  feet    in   diameter.       However,    of  these  sma11  Potted  trees  are  a  cen- 

the  distinction  between  herbs, 

shrubs,  and  trees  is  not  one  of  size.  Herbaceous  plants,  like 
the  corn  and  sunflower,  may  reach  a  height  of  over  15  feet 
and  the  banana  a  height  of  30  feet,  while  some  shrubs  are  only 
a  few  inches  in  height  and  some  of  the  dwarf  trees  of  Japan 
that  are  a  century  old  are  less  than  5  feet  in  height  (Fig.  60). 
Plant  characteristics  and  the  plant-producing  arts.  The 
differences  in  the  habits  of  growth,  longevity,  and  materials 
stored  by  plants  has  led  to  specialization  among  those  who 
grow  plants.  For  many  evident  reasons  the  most  important 
art  of  growing  plants  is  agriculture.  The  farmer  deals  entirely 
with  herbs  and  largely  with  annuals,  though  biennials  and 
perennials  may  be  grown  for  forage  crops.  He  is  for  the 


ioo  Science  of  Plant  Life 

most  part  concerned  with  plants  that  accumulate  foods  in  a 
highly  concentrated  form. 

The  growing  of  trees  to  create  forests  for  the  production  of 
timber,  fuel,  and  pulp  wood  is  the  field  of  silviculture.  The 
silviculturist  specializes  on  those  trees  that  accumulate  cel- 
lulose in  the  most  usable  form. 

Horticulture  embraces  a  wider  range  of  plants,  but  in  actual 
practice  a  horticulturist  usually  specializes  on  plants  having 
somewhat  similar  habits.  The  growing  of  food-producing 
shrubs  and  trees  represents  one  division  of  horticulture. 
The  object  sought  is  the  production  of  fruits  containing 
pleasantly  flavored  substances  stored  in  cells  with  the  thin- 
nest possible  cell  walls.  The  vegetable  grower  specializes 
on  annuals  and  biennial  herbs  that  accumulate  both  food 
and  flavors,  and  to  a  less  extent  on  perennials,  like  asparagus 
and  rhubarb.  Floriculture  deals  with  all  classes  of  plants 
and  has  for  its  object  the  production  of  attractive  flowers 
and  foliage.  It  reaches  its  highest  development  in  landscape 
architecture,  in  which  masses  of  vegetation  are  arranged  to 
beautify  a  landscape. 


Suggestions  for  Laboratory  and  Field  Wo*-k  -to. 
Chapter  Ten 

1.  Make  accurate  drawings  of  small  portions  of  herbaceous  and 
woody  stems,  noting  the   various   surface   features.     Goldenrod, 
sunflower,  aster,  gourd,  buckeye,  box  elder,  raspberry,  elm,  ailan- 
thus,  and  hawthorn  are  particularly  suitable.     Label  the  terminal 
buds,  lateral  buds,  leaf  scars,  bundle  scars,  lenticels,  and  bud  scars. 

2.  Study  branches  of  spruce  and  apple,  to  show  growth  of  dif- 
ferent years. 

3.  Place  some  twigs  of  soft  maple,  ailanthus,  willow,  and  cotton- 
wood  in  water.     Make  a  series  of  drawings  to  show  stages  in  the 
opening  of  the  buds. 

4.  Make  sketches  of  several  trees  to  show  the  deliquescent  and 
excurrent  types  of  branching. 


101 


.    CHAPTER   TEN 


STEMS  AND   THEIR  EXTERNAL  FEATURES 

THE  stem  usually  forms  the  axis  of  the  plant  and  bears  the 
leaves,  flowers,  and  fruits.  Plants  showing  all  degrees  of 
stem  branching  are  found,  from  the  unbranched  palm  and 
corn  to  the  finely  divided  asparagus  and  elm.  In  most 
plants  the  stems  are  upright,  aerial  structures ;  but  in 
some  plants  they  lie  on  the  surface  of  the  soil,  in  other  plants 
the  main  stem  is  underground  and  only  the  branches  rise 
above  the  surface,  and  still  other  plants  have  the  entire  stem 
underground.  The  upright  stem  is  the  common  type  and  has 
many  advantages  over  a  horizontal  stem.  At  its  lower  end 
it  bears  one  or  more  roots  that  connect  the  plant  with  the  soil. 

Advantages  of  upright  stems. 
The  photographer  uses  light  to 
effect  chemical  changes  in  pho- 
tographic papers  and  plates, 
and  the  plant  uses  light  to  bring 
about  chemical  changes  in  its 
green  tissues.  The  photog- 
rapher who  uses  sunlight  for 
his  work  usually  locates  his 
studio  at  the  top  of  a  tall  build- 
ing, because  there  he  avoids 
the  shadows  of  near-by  build- 
ings and  secures  a  more  con- 
stant exposure  to  light.  The 
same  advantages  come  to  the 
plant  that  has  its  leaves  raised 
well  above  surrounding  plants  : 
the  leaves  are  in  less  danger  of 


FIG.  61.  Sunflower  and  burdock,  show- 
ing advantage  of  upright  stem  in  leaf 
display. 


102 


Stems  and  Their  External  Features 


103 


being  shaded,  and  each  day 
they  are  exposed  to  the  sun- 
shine during  a  longer  period. 
The  tall  plant  has  an 
additional  advantage  in 
being  able  to  expose  to  the 
light  a  greater  leaf  area 
over  a  given  space  of 
ground,  because  it  can  dis- 
play several  or  many  layers 
of  leaves  one  above  the 
other.  The  rosette  of 
leaves  formed  by  the  bur- 
dock illustrates  the  possi- 
bilities of  leaf  display  near 
the  soil.  A  large  sunflower 
plant  covers  no  greater  soil 
area  than  a  burdock,  but 
it  is  able  to  expose  to  the 
sunlight  several  times  as 
great  a  leaf  area,  because 

the  sunflower  leaves  are  placed  at  several  different  levels  (Fig. 
6 1 ) .  Trees  have  the  greatest  stem  development  and  the  greatest 
leaf  display.  Rosette  plants,  like  the  dandelions  and  plantains, 
represent  the  opposite  extreme  of  slight  stem  development, 
small  leaf  area,  and  a  poor  leaf  display.  One  advantage  in  a 
tall,  upright  stem  is  that  it  holds  the  leaves  up  to  the  light  and 
thereby  makes  possible  a  greater  leaf  display. 

Plants  with  upright  stems  have  advantages  also  in  connec- 
tion with  reproduction  and  seed  dispersal.  Wind-pollinated 
flowers,  like  those  of  corn  or  the  pine,  are  better  exposed  to 


Edwin  Hale  Lincoln 

FIG.  62.  An  American  elm  in  the  Berkshire 
Hills,  Massachusetts.  The  tall  stem  of  such 
a  plant  makes  possible  the  display  of  a  large 
number  of  leaves  to  the  light  and  facilitates 
the  production  and  dispersal  of  seed. 


104  Science  of  Plant  Life 

air  currents  when  they  are  borne  on  an  upright  stem,  and  the 
display  of  insect-pollinated  flowers  high  up  on  a  stem  is 
advantageous  because  the  flowers  are  more  readily  found 
by  insects.  Moreover,  seeds  are  likely  to  be  distributed 
more  widely  when  they  fall  from  tall  plants.  For  example, 
a  maple  seed  dropped  from  a  height  of  50  feet  is  exposed 
to  air  currents  during  its  fall  and  is  almost  sure  to  reach 
the  ground  at  some  distance  from  the  point  directly  below 
the  starting  place.  A  second  advantage  in  an  upright  stem 
is  that  it  facilitates  the  production  and  dispersal  of  seed. 

The  advantages  of  the  upright  stem  are  all  dependent  on  its 
capacity  to  support  other  organs.  The  stem  must  be  strong 
enough  to  support  leaves,  flowers,  and  fruits.  The  city  sky- 
scraper needs  first  of  all  a  strong  framework  about  which  the 
building  is  constructed.  The  stems  of  tall,  erect  plants  must 
be  correspondingly  strengthened  by  a  mechanical  structure. 
The  base  of  a  tree  is  much  smaller  in  proportion  to  its  height 
than  that  of  the  tallest  and  narrowest  building,  and  it  is  pos- 
sible for  trees  to  reach  great  heights  only  because  their  stems 
are  composed  in  large  part  of  supporting  tissues  that  have 
great  strength.  Mechanical  or  supporting  tissues  are  neces- 
sary in  upright  stems. 

Advantages  of  horizontal  stems.  Horizontal  stems  have 
little  or  no  mechanical  tissue,  and  they  display  leaves  to  the 
light  advantageously  only  when  they  grow  in  the  open.  There 
are  advantages  in  stems  of  this  type,  however,  because  by 
growing  horizontally  on  the  soil  or  beneath  the  surface  of  the 
soil  they  spread  the  plant ;  because  they  are  in  contact  with 
the  soil  and  may  take  root  at  frequent  intervals ;  and  because 
they  are  better  protected  than  upright  stems  during  the  win- 
ter and  other  unfavorable  seasons. 


Stems  and  Their  External  Features  105 

Conductive  tissues  necessary  in  stems.  The  simplest  land 
plants  are  very  small  and  grow  flat  on  the  soil  in  wet  places. 
They  are  constantly  in  contact  with  the  moist  soil,  and  their 
cells  can  be  supplied  almost  directly  with  water  and  mineral 
salts.  In  such  plants  a  conductive  system  is  not  necessary ; 
but  if  the  leaves  of  a  plant  are  to  be  raised  into  the  air,  water 
for  transpiration  must  not  only  be  supplied  to  them  con- 
tinuously, but  at  times  it  must  be  supplied  in  great  quantity. 
Because  of  this  fact,  a  plant  that  raises  its  leaves  even  a  few 
inches  above  the  soil  must  possess  conductive  tissues,  and 
when  large  numbers  of  leaves  are  raised  200  or  300  feet  into 
the  air,  a  very  extensive  water-conducting  system  is  neces- 
sary. The  water-conducting  tissues  are  of  vital  importance  in 
the  stems  of  complex  plants. 

The  roots  and  stems  require  a  continuous  supply  of  food  for 
repairing  old  cells  and  for  building  new  ones.  Since  the  foods 
are  manufactured  primarily  in  the  leaves,  there  must  be  food- 
conducting  tissues  that  are  adequate  to  carry  them  to  all 
parts  of  the  stem  and  roots.  The  food-conducting  tissues 
also  transfer  food  from  the  leaves  to  the  seeds  and  growing 
parts,  and  when  food  has  accumulated  in  the  stem  or  roots 
it  may  pass  up  through  the  conductive  tissues  of  the  stem  to 
other  parts  of  the  plant.  Food-conducting  tissues  are  neces- 
sary in  stems  to  transfer  food  within  the  plants. 

The  stem  as  a  place  of  food  accumulation.  Because  of  the 
volume  of  the  stem  it  is  natural  that  excess  food  should  ac- 
cumulate in  it.  Under  favorable  conditions,  foods  for  which 
the  plant  has  no  immediate  need  are  continuously  passing 
from  the  leaves  into  the  stem.  Consequently,  in  our  larger 
plants  the  stem  is  the  place  of  temporary  storage  and  the  center 
from  which  foods  are  distributed  as  needed.  In  some  plants 


106  Science  of  Plant  Life 

the  process  of  accumulation  becomes  one  of  the  most  im- 
portant functions  of  the  stem,  and  it  may  include  the  ac- 
cumulation of  water  as  well  as  of  food  (page  75). 

Stems  as  photosynthetic  organs.  In  plants  with  little  or 
no  leaf  display,  the  stems  may  do  all  or  most  of  the  photo- 
synthetic  work.  The  night-blooming  cereus  and  other  cac- 
tuses, and  asparagus  and  equisetum,  have  no  leaves.  The 
green  stems  of  most  herbaceous  plants  contribute  at  least  a 
part  of  the  carbohydrates,  and  the  young  twigs  of  many 
woody  plants  are  green  and  carry  on  photosynthetic  work. 

Physiologically,  then,  the  stem  is  an  organ  that  supports 
and  displays  leaves  to  the  light ;  aids  reproduction  by  ele- 
vating the  flowers  and  seeds ;  conducts  water  to  the  leaves  for 
transpiration  and  photosynthesis ;  carries  food  to  its  own 
living  cells  and  to  those  of  the  roots ;  is  a  place  for  the  tem- 
porary accumulation  of  food  materials ;  and  may  carry  on 
photosynthesis. 

External  features  of  woody  stems.  On  a  woody  stem  nodes, 
leaf  scars,  buds,  and  lenticels  may  be  seen.  The  nodes  are  the 
places  where  the  leaves  arise,  and  they  are  the  most  prominent 
external  feature  of  stems.  The  arrangement  of  leaves  at  the 
nodes  has  already  been  discussed  (page  37).  In  addition  to 
the  leaf,  the  node  gives  rise  to  one  or  more  buds,  just  above 
the  place  of  leaf  attachment,  in  the  so-called  axil  (Latin : 
axilla,  armpit)  of  the  leaf.  The  leaf  scars  are  markings  on  the 
stem  where  leaves  have  fallen.  The  part  of  a  stern  between 
two  nodes  is  called  an  internode.  The  lenticels  are  small, 
dotlike  elevations  scattered  over  the  surfaces  of  the  internodes. 

Buds.  Stems  and  branches  produce  leaves  only  once.  We 
are  accustomed  to  speak  of  deciduous  trees  clothing  themselves 
with  a  new  set  of  leaves  each  spring,  as  though  the  branches 


Stems  and  Their  External  Features 


107 


of  the  previous  year  put  forth  a  new  set  of  leaves  to  replace 

those  lost  the  preceding  autumn.     As  a  matter  of  fact,  when 

we  look  at  a  deciduous  tree 

in  winter,  we  see  branches 

and  twigs,  all  of  which  have 

borne   leaves   and  none  of 

which  will  ever  bear  leaves 

again.      The   possibility  of 

producing  new  foliage  lies 

in  the  development  of  new 

branches  and  twigs.     This 

is  the  function  of  the  buds ; 

from  them  the  new  growth 

of  each  year  takes  its  rise. 

The  buds  of  many  tropi- 
cal plants  are  like  those  we 
see  at  the  tops  of  the  stems 
of  garden  vegetables.  Such 
a  bud  consists  of  the  stem's 
growing  point  and  the  un- 
developed leaves,  with  no 
special  coverings  of  any 
kind.  These  naked  buds 
occur  also  on  the  under- 
ground stems  of  some  of 
our  Northern  plants.  A 
simple  sort  of  bud  covering, 
which  is  common  in  the  FlG  63  Twigs  of  smilax  M)>  buckeye  (B}> 

tropics,  is  made  by  the  fold-    and    tree-of -heaven    (Ailanthus)    (C).      The 

ing  together  of  the  Stipules.     ^  SCalCS  are Designated  by  «;   b  and  h  are 

°4  *     ^          buds;   c  is  a  leaf  scar,  d  a  bundle  scar,  e  a 

This    type   Of    bud    Covering    lenticel,  /  a  terminal  bud  scar,  and  g  a  tendril 


io8  Science  of  Plant  Life 

may  be  seen  in  the  tulip  tree  and  the  magnolias  of  temperate 
climates.  The  buds  of  most  temperate  perennials  are  covered 
with  scales.  Not  infrequently  the  scales  are  further  covered 
with  matted  hairs  and  secretions  of  wax  and  resin.  These  all 
tend  to  make  the  bud  coverings  impervious  to  water.  In  this 
way  the  tender  growing  parts  are  protected  from  excessive  loss 
of  water  during  the  winter  and  during  the  still  more  critical 
stage  in  early  spring  when  the  buds  are  opening. 

We  are  likely  to  think  of  buds  as  being  formed  at  about  the 
time  when  the  leaves  fall  from  the  trees.  A  good  observer, 
however,  will  have  noted  that  the  buds  begin  to  develop  when 
the  leaves  unfold  in  spring,  and  that  they  grow  all  summer 
long.  Because  of  the  prominence  of  the  leaves,  the  buds  are 
obscured  somewhat  during  the  summer  months,  and  become 
conspicuous  only  after  the  leaves  are  gone, 

The  opening  of  buds.  When  the  warm  weather  of  spring- 
time comes,  the  innermost  bud  scales  begin  to  grow  and 
expand.  Sometimes  the  outer  scales  are  pushed  off ;  some- 
times they  elongate  and  grow  like  the  inner  ones.  But  the 
scales  quickly  reach  their  full  growth,  and  soon  they  are  cut 
off  by  the  formation  of  an  abscission  layer  at  the  base  of  each. 
In  the  buds  of  a  few  plants  all  the  scales  are  dead  and  are 
pushed  off  by  the  growth  of  the  stem  and  leaves  inside.  The 
expansion  of  bud  scales  and  leaves  takes  place  almost  wholly 
through  the  enlargement  of  cells  'already  formed.  Within 
the  bud  the  minute  leaf  cells  absorb  water  and  develop  large 
vacuoles.  The  expansion  of  these  cells  results  in  the  en- 
largement and  spreading  of  the  leaves.  Material  for  the  study 
of  the  different  habits  of  bud  expansion  may  be  secured  in 
winter  by  bringing  branches  of  different  kinds  of  trees  into  a 
warm  room  and  placing  them  in  water  until  the  leaves  expand. 


Stems  and  Their  External  Features  109 

Kinds  of  buds.  Every  bud  contains  the  growing  point  of  a 
stem.  In  addition,  most  buds  contain  the  beginnings  of 
foliage  leaves ;  that  is,  the  leaves  have  already  begun  to  de- 
velop on  the  sides  of  the  young  stem  within  the  bud.  Some 
buds,  as  for  example  many  of  those  on  the  maples  and  elms, 
contain  the  beginnings  of  flowers.  Other  buds,  like  some  of 
those  of  the  catalpa  and  the  horse-chestnut,  contain  both 
leaves  and  a  flower  cluster.  Bulbs  are  really  a  special  under- 
ground form  of  bud,  and  they  are  similar  in  structure  to  other 
buds.  We  shall  consider  bulbs  when  we  come  to  the  study 
of  underground  stems. 

Bud  development  and  plant  form.  Buds  which  occur  at 
the  ends  of  stems  are  called  terminal  buds  ;  those  which  occur 
at  the  nodes  are  called  lateral  buds.  This  classification  is 
useful  because  only  a  part  of  the  buds  on  a  stem  ever  de- 
velop and  because  the  form  of  a  plant  depends  on  which  set 
of  buds  develops  more  freely  and  grows  more  rapidly.  In 
most  plants,  the  terminal  bud  simply  extends  a  stem  or 
branch ;  the  lateral  buds  produce  new  branches.  Plants 
with  very  strong  terminal  buds  tend  to  become  columnar  in 
form,  like  the  large,  unbranched  sunflowers  of  the  garden  or 
like  the  spruce  and  palm  among  trees.  Plants  with  strong 
lateral  buds  tend  to  branch  continually  and  to  become  bushy 
in  form,  like  the  lilac  and  hydrangea.  There  are  all  grada- 
tions between  these  extremes  in  the  development  of  the 
terminal  and  lateral  buds,  and  in  the  resulting  plant  forms. 

In  many  roses  the  shoots  from  the  base  of  the  stem  develop 
only  through  their  terminal  buds  the  first  year.  The  shoot  is 
thus  extended  to  great  length  by  the  first  season's  growth. 
The  following  year  the  lateral  buds  develop,  and  the  long 
shoot  becomes  highly  branched.  As  these  lateral  branches 


no 


Science  of  Plant  Life 


FIG.  64.  Date  palms  in  fruit,  on  an  oasis  in  an  Algerian  desert.  The  terminal 
bud  continues  its  development  year  after  year  and  builds  up  the  long,  un- 
branched  stem  of  the  tree.  (From  photo  U.  S.  Dept.  of  Agriculture) 


Stems  and  Their  External  Features  m 


FIG.  65.     Pine  trees  on  Wood  River  in  Oregon.     The  strong  terminal  bud 
continues  its  development,  and  the  excurrent  stem  is  the  result. 

bear  the  flowers  and  produce  them  abundantly  only  once,  we 
can  promote  flowering  in  these  roses  by  trimming  away  each 
year  all  but  the  long,  unbranched  shoots.  In  many  other 
shrubs,  as  spiraea,  barberry,  and  privet,  a  few  strong  lateral 
buds  at  the  surface  of  the  soil  develop  each  year.  This  ac- 
counts for  the  basal  branching  of  these  plants. 

Excurrent  and  deliquescent  stems.  When  trees  have 
strong  terminal  buds,  the  main  stem  extends  to  the  top  and 
is  called  excurrent  (Latin:  excurrens,  running  out).  The 
spruce  has  a  strong  terminal  bud,  and  just  beneath  it  several 
smaller  lateral  buds.  The  terminal  bud  grows  upward,  and 
the  lateral  buds  grow  outward,  forming  a  whorl  of  branches 
at  the  base  of  the  season's  growth.  This  is  repeated  each 
year,  the  terminal  shoot  lengthening  the  stem,  and  the  lateral 
buds  adding  a  new  whorl  of  branches.  Consequently  each 


112 


Science  of  Plant  Life 


,v 


FIG.  66.  A  hackberry  in  Illinois.  The  de- 
velopment of  the  lateral  buds  results  in  the 
formation  of  a  deliquescent  stem. 


year's  growth  is  marked 
by  a  whorl  of  branches, 
and  the  age  of  a  tree  may 
readily  be  estimated  by 
counting  the  number  of 
whorls  on  the  stem.  Since 
the  oldest  branches  are 
nearest  the  ground  they 
are  the  longest,  and  the 
tree  becomes  cone-shaped 
as  it  grows. 

The  terminal  buds  of  the 
elm  tree  seldom  survive  the 
winter.  The  lateral  buds 
develop,  and  the  main  stem 
divides  and  subdivides  un- 
til it  is  lost  in  the  crown 


of  the  trees.  This  gradual  dissolving  of  the  trunk  into  a 
spray  of  terminal  branchlets  suggested  the  name  deliquescent 
(Latin :  deliquescens,  dissolving)  for  this  type  of  stem.  We 
see,  therefore,  that  the  excurrent  type  of  stem  depends  on 
the  continual  development  of  terminal  buds,  while  the  deli- 
quescent type  depends  on  the  growth  of  lateral  buds.  Con- 
sequently we  may  modify  the  forms  of  plants  in  cultivation 
by  trimming  them  and  so  forcing  the  growth  of  certain  buds. 
Lawn  trees  and  shrubs  are  grown  either  for  shade  or  for 
ornamental  effects.  We  secure  shade  by  trimming  off  the 
terminal  buds  and  so  causing  many  of  the  lateral  buds  to 
develop  into  branches  and  thus  form  a  denser  crown.  Orna- 
mental effects  are  secured  by  trimming  plants  so  that  they 
will  be  in  artistic  harmony  with  their  surroundings. 


Stems  and  Their  External  Features  113 


U.  S.  Dept.  of  Agriculture  (J.  Craig) 

FIG.  67.     Baldwin  apple  orchard,  showing  trees  with  the  branches  thinned  to 
increase  the  production  of  fruit. 

Fruit  trees  and  grapes  have  been  found  to  produce  more 
fruit,  and  fruit  of  a  better  quality,  when  the  number  of  branches 
is  small.  A  smaller  number  of  branches  on  a  tree  secures  an 
open  crown  and  permits  the  sunlight  to  penetrate  to  every 
leaf,  and  the  removal  of  some  of  the  branches  forces  the  de- 
velopment of  flower  buds  which  might  remain  dormant  if 
the  terminal  and  branch  buds  were  allowed  to  grow  uninter- 
ruptedly. In  grape  culture,  only  four  or  five  branches  are 
allowed  to  remain  on  a  vine  each  year,  and  these  branches  are 
shortened.  This  insures  full  development  for  a  few  of  the 
flowering  branches  and  the  production  of  the  best  quality  of 
fruit. 

Leaf  scars  and  bud  scars.  The  leaf  scars  on  some  plants 
are  round ;  on  others  they  are  narrow  lines ;  on  most  plants 
they  are  crescent-shaped.  Usually  they  are  smooth,  except 


114 


Science  of  Plant  Life 


for  small,  dotlike  markings.  These  markings  are  bundle 
scars;  they  show  where  the  bundles  of  conductive  and  me- 
chanical tissue  extended  outward  from  the  stem  into  the 
petiole.  The  shape  of  the  leaf  scar  and  the  arrangement  of 
the  bundle  scars  are  so  characteristic  for  many  kinds  of  trees 
that  they  may  serve  to  identify  the  tree  in  winter. 

The  bud  scales  also  leave  scars  when  they  drop.  These 
scars  are  usually  numerous  and  so  closely  crowded  that  they 
form  a  roughened  ring  about  the  stem.  The  terminal-bud 
scars  occur  at  intervals,  surrounding  the  stem  or  branch.  The 
lateral-bud  scars  occur  only  at  the  bases  of  the  branches  and 
twigs. 

Determining  annual  growth  of  shoots  from  terminal-bud 
scars.  Since  the  terminal  bud  marks  the  end  of  each  year's 

growth,  the  terminal-bud  scars 
mark  off  a  perennial  stem  into 
segments,  each  of  which  repre- 
sents the  growth  of  a  single 
year.  Often  an  interesting 
life  history  is  suggested  by  the 
varying  length  of  the  intervals 
between  the  bud  scars  on  a 
particular  stem.  By  a  study 
of  these  intervals  we  can  de- 
termine the  seasons  that  were 
favorable  and  those  that  were 
unfavorable  on  account  of 
drought,  excessive  rain,  attacks 
of  insects,  or  some  other  cause. 
In  the  pines  and  spruces  the 


FIG.  68.    Norway  spruce,  showing  whorls  . 

of  branches  at  end  of  each  year's  growth,     years    growth    IS    marked     Ott 


Stems  and  Their  External  Features  115 

not  only  by  the  bud  scars,  but  also  by  whorls  of  branches. 
Differences  in  the  color  of  the  bark  and  in  its  texture  will  also 
help  to  distinguish  successive  annual  stem  segments  in  most 
trees. 

Lenticels.  All  living  cells  require  energy.  This  is  largely 
obtained  from  respiration.  Therefore,  in  addition  to  a  con- 
stant food  supply,  the  cells  of  the  stem  must  have  access  to 
oxygen.  As  in  the  leaves  the  oxygen  is  supplied  through  the 
intercellular  spaces,  so  in  stems  there  must  be  sufficient  in- 
tercellular spaces  to  permit  oxygen  to  diffuse  inward  and 
carbon  dioxid  to  diffuse  outward.  There  must  also  be  open- 
ings through  the  epidermis  or  bark  to  connect  these  inter- 
cellular spaces  with  the  outside  atmosphere. 

The  young  green  stems  of  all  plants  have  stomata.  Peren- 
nial stems,  however,  soon  develop  a  corky  layer  beneath  the 
epidermis,  which  cuts  the  cells  in  the  interior  of  the  stem  off 
from  the  stomata.  While  this  layer  is  developing,  masses  of 
round,  loose  cells  form  beneath  some  of  the  stomata,  pushing 
out  and  tearing  the  epidermis  above  them.  These  open  places 
are  the  lenticels.  They  permit  gas  exchanges,  and  in  older 
stems  take  the  place  of  the  stomata.  The  lenticels  of  most 
trees  and  shrubs  are  closed  in  the  late  autumn  by  the  growth 
of  a  thin  layer  of  cork  beneath  them.  The  following  spring 
loose  cells  are  again  formed  at  the  same  point,  the  cork  is 
burst  open,  and  the  lenticels  again  permit  gas  exchanges. 

In  the  cherry  and  birch  the  lenticels  persist  for  many  years 
and  become  elongated  transversely,  forming  granular  rings 
part  way  around  the'  stem.  In  the  trunks  of  thick-barked 
trees,  the  lenticels  occur  in  the  furrows  of  the  bark. 


n6  Science  of  Plant  Life 

PROBLEMS 

1.  What  advantage  in  resisting  wind  have  tall,  columnar  tree  trunks  over 
equally  tall  smokestacks  or  monuments?     What  disadvantage? 

2.  What  are  the  perennial  food-producing  plants  of  your  locality? 

3.  Find  out  how  your  local  gardeners  trim  their  grapevines,  berry  bushes,  and 
fruit  trees.     Secure  definite  information  for  five  of  these  plants,  and  deter- 
mine the  reasons  underlying  the  practices. 

4.  What  are  the  best  trees  for  street  planting  in  your  locality?     What  trees 
now  planted  there  are  objectionable?     Why? 

5.  Compare  a  tree  growing  in  an  open  field  with  one  of  the  same  species  grow- 
ing in  the  woods.     Account  for  the  differences  in  arrangement  of  branches 
and  leaves. 

6.  Which  will  furnish  the  better  lumber,  a  tree  grown  in  the  open,  or  one  grown 
in  the  forest?     Why? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Eleven 

1.  Sketch  cross  sections  of  some  monocot  stems  like  corn, 
bamboo,  asparagus,  lily,  or  wheat.    Note  particularly  the  arrange- 
ments of  the  bundles.     Cut  a  longitudinal  section  through  a  node, 
and  note  how  the  bundles  unite  in  the  node. 

2.  Sketch  a  cross  section  of  a  peach  branch  that  is  an  inch  or 
two  in  diameter.     Note  pith,  wood,  bark,  cortex,  cortical  paren- 
chyma, cork,  cambium,  annual  rings,  and  pith  rays. 

3.  Sketch  a  cross  section  of  a  pine  stem  and  note  the  parts,  as 
in  the  peach. 

4.  With  a  microscope,  examine  thin  sections  of  monocot,  dicot, 
and  conifer  stems,  and  note  differences  in  the  structures  of  the 
bundles. 

5.  Examine  sections  of  tree  trunks  and  determine  the  number  of 
annual  rings  in  the  sap  wood  and  in  the  heart  wood.     What  fraction 
of  a  large  tree  trunk  is  alive  ? 

6.  Bring  a  branch  of  a  tree  to  the  laboratory  and  try  to  set  up 
cleft  and  whip  grafts.     Also  try  to  set  a  bud.     Repeat  the  opera- 
tion on  trees  out  of  doors  and  see  whether  or  not  the  cions  will  live. 

7.  Show  the  region  of  growth  by  marking  the  upper  end  of  a 
stem  with  transverse  lines  at  equal  intervals. 

8.  Show  the  effect  of  gravity  on  the  direction  of  growth  by 
changing  the  position  of  a  growing  stem  with  reference  to  gravity. 

9.  Show  the  effect  of  light  on  the  direction  of  stem  growth  by 
placing  a  plant  where  light  can  reach  it  from  only  one  direction. 


117 


CHAPTER   ELEVEN 

THE   STRUCTURES   AND   PROCESSES    OF   STEMS 

IF  we  study  the  development  of  a  stem  from  a  bud,  we  find 
that  the  growing  point  is  made  up  of  very  minute  cells,  all  of 
which  are  practically  alike.  These  cells  divide  to  make  other 
cells  like  themselves,  and  the  lower  ones  begin  to  enlarge.  In 
this  way  the  growing  point  is  pushed  forward  and  the  di- 
ameter of  the  stem  increased.  Upon  these  two  processes,  cell 
division  and  cell  enlargement,  the  growth  of  the  stem  depends. 
Then  certain  groups  of  cells  begin  to  take  on  special  forms. 
The  cells  that  are  to  form  the  bundles  elongate ;  some  of  them 
develop  woody  walls.  Others  elongate  but  remain  thin- walled, 
and  these  form  the  food-conducting  tissue.  The  other  tis- 
sues of  the  stem  are  composed  of  cells  which  have  enlarged 
and  have  become  rounded  or  variously  angled,  and  which 
have  their  walls  more  or  less  thickened.  These  cells  form  the 
pith  or  soft  inner  part  of  the  stem,  and  the  cortex  or  outer 
portion.  In  this  way  the  various  tissues  of  stems  arise  from 
the  small  uniform  cells  of  the  growing  point. 

Stem  structures  and  plant  groups.  There  are  three  groups 
of  seed  plants  that  we  wish  to  distinguish  at  this  time  because 
the  stems  of  the  plants  that  belong  to  these  groups  differ 
fundamentally.  These  groups  are  :  (i)  the  conifers,  or  cone- 
bearing  trees,  like  pines,  spruces,  firs,  and  cedars,  that  have 
scale  or  needle  leaves  and  are  for  the  most  part  evergreen; 
(2)  the  monocotyledonous  plants  (monocots),  or  plants  with 
parallel- veined  leaves,  like  the  grasses,  lilies,  cannas,  orchids, 
and  palms;  and  (3)  dicotyledonous  plants  (dicots),  or  plants 
with  net- veined  leaves,  like  oaks,  maples,  sunflowers,  asters, 
and  clovers. 

The  stems  of  the  plants  belonging  to  these  three  groups 

118 


The  Structures  and  Processes  of  Stems         119 


Epidermis  and 
cuticle 


FIG.  69.     Stem  of  moonseed  vine,  showing  tissues  and  their  arrangement. 


120  Science  of  Plant  Life 

differ  in  (i)  the  kinds  of  tissue  making  up  the  bundles,  and 
(2)  the  arrangement  of  the  bundles  in  the  stem.  We  shall 
first  study  the  bundles  and  the  arrangement  in  a  dicot  stem, 
and  then  we  shall  learn  how  the  stems  of  monocots  and 
conifers  differ  from  those  of  dicots. 

The  structure  of  a  dicot  stem.  When  a  dicot  stem  is  cut 
across,  the  bundles  are  seen  to  be  arranged  in  a  ring.  The 
cylinder  of  tissue  lying  inside  the  bundle  cylinder  is  the  pith ; 
outside  the  bundle  cylinder  is  the  cortex ;  and  covering  the 
cortex  is  an  epidermis  very  similar  to  that  of  leaves  (Fig.  69). 
In  older  and  harder  stems  the  outer  cortical  cells  have  thick 
walls  and  form  a  corky  or  hard  outer  covering  that  replaces 
the  epidermis.  The  pith  and  the  inner  part  of  the  cortex 
are  made  up  of  rounded,  thin- walled  cells  called  parenchyma. 
In  annuals  and  young  perennials  the  cortical  parenchyma  con- 
tains chlorophyll  and  resembles  the  mesophyll  of  the  leaf  in 
appearance  and  function.  It  is  this  tissue  that  forms  the 
inner  "  green  bark  "  of  twigs  and  gives  the  green  color  to  the 
stems  and  branches  of  herbaceous  plants. 

There  are,  then,  four  distinct  layers  in  dicot  stems :  (i)  on 
the  outside  is  the  epidermis;  (2)  from  the  epidermis  to  the 
bundles  is  the  cortex;  (3)  inside  the  cortex  is  the  hollow 
bundle-cylinder ;  (4)  the  pith  forms  the  axis  of  the  stem,  fill- 
ing the  space  inside  the  cylinder  of  bundles  (Fig.  69) . 

Between  the  bundles  of  the  dicot  stem  there  are  strands  of 
parenchyma  cells  that  connect  the  pith  parenchyma  with  the 
cortical  parenchyma.  These  are  the  pith  rays.  They  con- 
vey food  across  the  stem,  and  with  the  other  parenchyma 
cells  form  a  complex  tissue  system  in  which  excess  foods 
accumulate  and  from  which  they  later  move  to  other  parts  of 
the  plant. 


The  Structures  and  Processes  of  Stems         121 

General  structure  of  the  dicot  bundle.  The  bundles  in  a 
plant  stem  terminate  above  in  the  veins  of  the  leaves,  and 
below  they  connect  with  the  bundles  of  the  roots.  .  In  the 
dicot  stem  these  bundles  contain  four  tissues :  (i)  the  water- 
conducting  tissue,  (2)  the  food-conducting  tissue,  (3)  the 
cambium,  and  (4)  the  mechanical  tissue.  The  cambium  is  a 
layer  of  thin-walled  cells  that  lies  lengthwise  in  the  bundle  and 
separates  the  inner  water-conducting  tissue  from  the  outer 
food-conducting  tissue. 

Tissues  of  the  dicot  bundle.  The  water-conducting  tissue 
contains  long,  tubelike  vessels  made  up  of  cylindrical  cells 
joined  end  to  end,  often  for  considerable  distances  without 
end  walls  between  them.  These  tubes  (tracheae)  usually 
have  heavy  walls  marked  by  spiral  and  lattice-form  thicken- 
ings. When  mature  they  are  empty  of  protoplasm.  In 
other  words,  they  are  the  coverings  of  dead  cells  joined  to- 
gether to  form  tubes  usually  several  inches,  more  rarely  several 
feet,  in  length.  Mixed  with  them  are  smaller  and  shorter 
tubes,  and  cylindrical  cells  that  retain  the  cell  contents. 
All  together  these  tissues  form  the  passageway  for  the  move- 
ment of  water  and  mineral  salts,  and  sometimes  sugar,  to  all 
parts  of  the  plant.  The  general  direction  of  the  water  move- 
ment in  this  tissue  is  upward,  because  the  lifting  of  the  water 
is  brought  about  principally  by  transpiration  from  the  leaves 
(page  134). 

The  food-conducting  tissue  differs  from  the  water-conducting 
tissue  in  being  composed  of  smaller,  thin-walled  cells,  all  of 
which  retain  their  living  protoplasm.  The  largest  of  these 
cells  are  set  end  to  end,  and  the  end  walls  have  holes  in  them 
like  the  top  of  a  salt  shaker.  These  rows  of  cells,  therefore, 
form  tubes  with  sievelike  cross  walls  in  them,  and  on  this 


122 


Science  of  Plant  Life 


FIG.  70.  Cross  section  of  a  portion  of 
rootstock  of  calamus,  photographed 
through  a  microscope.  The  circular 
areas  are  the  cells  which  are  filled  with 
starch. 


account  they  are  called  sieve 
tubes.  Through  the  openings 
in  the  sieve  plate  the  proto- 
plasm is  continuous  from  cell  to 
cell,  and  through  these  tubes 
the  foods  pass  from  one  part 
of  the  plant  to  another.  Be- 
cause the  cells  of  the  stem  and 
root  are  supplied  with  food 
manufactured  in  the  leaves,  it 
is  often  said  that  the  move- 
ment of  foods  is  downward  in 
a  plant.  In  reality,  the  direc- 
tion of  the  food  current  is  not 
so  fixed  as  is  that  of  the  water 

current.  Food  moves  toward  any  part  of  the  plant  where  it 
is  being  used  or  is  being  accumulated.  For  example,  in  mid- 
summer when  a  tree  is  in  full  leaf  and  the  season's  growth 
has  practically  been  completed,  food  moves  out  of  the  smaller 
branches  into  the  larger  branches  and  the  trunk.  In  the 
spring,  when  leaves  and  blossoms  are  developing,  food  is 
being  used  in  the  twigs,  and  the  direction  of  the  movement 
of  food  materials  is  reversed. 

The  mechanical  tissue  is  made  up  of  cylindrical  or  spindle- 
shaped  cells  with  very  heavy  walls.  Indeed,  the  walls  at 
maturity  may  be  so  thick  as  to  render  the  cells  almost 
solid.  Ordinary  cellulose  is  not  very  hard,  but  the  walls 
of  the  mechanical  tissue  are  hardened  and  thickened 
by  a  deposit  of  a  substance  called  lignin.  The  differ- 
ence between  hard  and  soft  woods  is  for  the  most  part 
due  to  the  thickening  of  the  walls  of  the  mechanical  cells ; 


The  Structures  and  Processes  of  Stems         123 


secondarily  it   is   due   to  changes    in  the   walls  themselves 
(lignification) . 

Annual  rings 

Tracheae 


Pith 


Pith 


ravs 


FIG.  71.     Block  of  oak  wood  magnified  to  show  the  arrangement  of  the  various 
tissues  which  produce  the  patterns  on  polished  wood  surfaces.     (Diagrammatic.) 

Mechanical  tissue  is  found  on  both  the  water-conducting 
and  food-conducting  sides  of  the  bundle.  On  the  food-con- 
ducting side  it  lies  outside  the  food-conducting  tissue,  and  is 
made  up  of  long,  exceedingly  slender,  nearly  solid,  spindle- 
shaped  cells.  These  cells  are  called  bast  fibers,  and  the  tissue 


124 


Science  of  Plant  Life 


bamboo  stem  photographed  through  a 
microscope.  The  large  openings  in  each 
bundle  are  the  water-conducting  tubes. 


that  is  made  up  of  them  is 
called  the  bast.  Bast  may  be 
seen  in  the  stringy  fibers  on  a 
grapevine  or  in  the  bark  of 
trees.  It  is  the  bast  fibers 
from  flax,  hemp,  jute,  and 
other  dicotyledonous  plants 
that  are  used  in  the  manufac- 
ture of  thread  and  cordage. 

The  cells  of  the  mechanical 

FIG.  72.   Cross  section  of  a  portion  of    tissue  on  the  water-conducting 

side  of  the  bundle  are  some- 
what shorter  and  thicker  than 
the  bast  fibers.  They  are 

known  as  wood  fibers  and  make  up  what  is  properly  called 
the  wood,  although  in  most  dicots  the  wood  fibers  are  mixed 
with  the  water-conducting  vessels,  and  the  whole  inner  part 
of  the  bundle  is  known  as  wood.  In  woody  plants  this  me- 
chanical tissue  is  present  in  abundance  and  forms  the  bulk 
of  the  stem.  The  lumber  that  is  obtained  from  dicotyledon- 
ous trees  is  derived  from  the  inner  parts  of  the  bundles  and 
is  made  up  of  wood  fibers  and  water-conducting  tissues. 

The  cambium  is  a  layer  of  soft  tissue  between  the  two  sides 
of  the  bundle.  It  is  the  principal  growing  tissue  of  the  dicot 
stem.  Growth  takes  place  in  it  by  the  longitudinal  division 
of  the  cells.  On  its  inner  face  the  cells  of  the  cambium  layer 
change  into  water-conducting  cells  or  wood  fibers ;  on  its 
outer  face  they  change  into  food-conducting  cells  or  bast 
fibers.  In  this  way  the  bundles  of  perennial  dicots  enlarge 
from  year  to  year,  and  this  causes  the  stem  to  increase  in 
thickness.  In  a  tree  the  cambium  cells  form  a  continuous 


The  Structures  and  Processes  of  Stems 


FIG.  73.  Cross  section  of  portion  of  a 
rattan  stem  photographed  through  a 
microscope.  Note  the  ring  of  bundles 
near  the  outside,  with  the  heavy-walled 
mechanical  tissue  and  the  scattered 
bundles  within. 


layer  between  the  wood  and 
the  bark,  and  the  diameter  is 
increased  by  the  addition  of 
successive  layers  of  tissues 
built  by  these  cells. 

Every  one  who  has  made 
willow  or  hickory  whistles  has 
become  acquainted  with  the 
cambium.  In  early  spring  the 
cambium  cells  are  dividing 
actively,  and  the  cambium 
layer  can  be  broken  by  tap- 
ping on  the  bark.  The  whole 
bark  can  then  be  readily 
stripped  from  the  wood. 

The  monocot  stem.  The  monocot  stem,  like  dicot  and 
conifer  stems,  is  bounded  externally  by  an  epidermis  which 
closely  resembles  that  of  the  leaf.  The  groundwork  of  the 
stem  is  made  up  of  parenchyma,  which  is  commonly  called 
the  pith.  The  parenchyma  is  usually  composed  of  thin- 
walled  cells,  and  is  the  principal  tissue  for  the  temporary  ac- 
cumulation of  foods ;  from  it  the  sugar  solution  is  obtained 
when  the  stems  of  sorghum  and  sugar  cane  (Fig.  48)  are  crushed. 
In  a  monocot  stem  the  bundles  are  scattered,  instead  of  being 
arranged  in  a  cylinder  as  they  are  in  a  dicot  stem.  In  the 
hollow  stems  of  grasses  they  are  scattered  through  the  cyl- 
inder of  parenchyma  tissue ;  in  a  cornstalk,  a  shoot  of  as- 
paragus, or  the  trunk  of  a  palm  they  are  distributed  through 
the  whole  stem.  As  in  the  dicot  and  conifer  bundles,  the 
water-conducting  tissue  is  on  the  side  next  the  center  of  the 
stem,  and  the  food-conducting  tissue  is  on  the  side  toward 


126 


Science  of  Plant  Life 


the  epidermis.     The  scattered  arrangement  of  the  bundles 
in  the  pith  may  easily  be  seen  in  a  stalk  of  corn. 


FIG.  74.  Plantation  of  abaca,  a  form  of  banana,  from  the  petioles 
of  which  Manila  fiber  is  obtained.  Abaca  flourishes  only  in  the 
Philippines.  The  fiber  is  used  chiefly  in  the  manufacture  of  ropes. 

The  monocot  bundle.  The  monocot  bundle  differs  from  the 
dicot  bundle  in  that  it  lacks  a  cambium  layer.  It  is  frequently 
called  a  closed  bundle,  because  in  the  absence  of  cambium 


The  Structures  and  Processes  of  Stems        127 


Bureau  of  Agriculture,  P.  I. 

FIG.  75.  Stripping  abaca  for  fiber.  The  long  petioles  are  pulled  under  toothed 
knives  which  scrape  the  soft  tissues  from  the  bundles.  Abaca  is  a  monocot,  and 
the  fiber  is  composed  of  an  entire  bundle. 

tissue  the  bundle  cannot  increase  in  size,  and  there  can  be  no 
growth  of  the  monocot  stem  through  the  multiplication  of 
cambium  cells.  The  dicot  bundle,  on  the  other  hand,  is 
spoken  of  as  open,  because  there  is  a  cambium  layer  between 
its  water-conducting  and  food-conducting  tissues  and  the 
bundle  can  increase  in  size.  The  monocot  bundle  differs 
further  from  the  dicot  bundle,  in  that  its  mechanical  tissues 
form  a  complete  sheath  about  the  food  and  water  conducting 
parts.  It  is  as  though  the  bast  of  the  outer  part  of  the  dicot 
bundle  and  the  wood  of  the  inner  part  were  joined  at  the  sides 
of  the  bundle  to  make  a  sheath  about  the  conducting  tissues. 
The  fibers  like  sisal  and  Manila  hemp,  that  are  derived 
from  the  monocots,  are  usually  coarser  than  the  fibers  derived 
from  dicots,  because  the  monocot  fibers  are  entire  bundles, 


128  Science  of  Plant  Life 

while  the  dicot  fibers  are  made  up  of  only  the  strand  of  bast 
cells  from  the  food-conducting  side.  The  bundle  sheaths 
are  usually  thicker  in  the  bundles  near  the  outer  part  of  the 
monocot  stem.  In  fact,  in  some  monocots  like  rattan  and 
bamboo,  the  sheaths  of  adjacent  outer  bundles  may  join  each 
other  and  thus  form  a  hard  layer  beneath  the  epidermis 

(Fig.  73)- 

The  structures  of  conifer  stems.  The  conifers,  like  the  di- 
cots,  have  their  bundles  arranged  in  a  hollow  cylinder.  In 
structure  these  bundles  are  somewhat  similar  to  those  of 
dicots,  except  that  the  wood  and  water-conducting  cells  are 
not  distinct.  The  wood  cells  form  the  water-conducting  tis- 
sue as  well  as  the  mechanical  tissue.  In  keeping  with  their 
double  function,  the  cells  (tracheids)  are  thick-walled  and 
spindle-shaped,  with  numerous  thin  places,  or  pits,  in  two  of 
the  walls.  Because  of  this  structure,  the  stem  retains  its 
rigidity  and  still  permits  the  ready  passage  of  water  and  min- 
eral salts. 

Growth  of  stems.  The  limit  of  growth  of  a  stem  is  not  so 
definite  as  that  of  leaves.  The  length  and  the  diameter  of 
a  stem  depend  largely  upon  the  conditions  under  which  the 
plant  lives,  the  available  water  supply,  amount  of  light, 
temperature,  and  quality  of  the  soil.  Along  a  dry  roadside 
a  ragweed  may  complete  its  development  with  a  stem  less 
than  6  inches  long,  while  in  a  rich  bottom-land  field  the  same 
plant  might  have  reached  a  height  of  15  feet. 

The  growth  in  length  takes  place  at  the  apex  of  a  stem,  the 
growing  point  being  located  in  the  terminal  bud.  The  grow- 
ing region  extends  back  from  the  tip,  sometimes  for  only  a 
fraction  of  an  inch,  more  rarely,  as  in  rapidly  growing  vines, 
a  foot  or  two.  If  we  mark  the  upper  portion  of  a  growing 


The  Structures  and  Processes  of  Stems         129 

stem  into  equal  spaces,  we  may  observe  on  the  following  day 
that  the  uppermost  space  has  elongated  the  most.  The  ad- 
joining spaces  below  are  less  and  less  elongated.  This  indi- 
cates that  the  greater  part  of  cell  division  and  enlargement 
takes  place  very  near  the  tip  (the  growing  point),  but  that 
some  growth  takes  place  in  the  cell  layers  for  a  certain  dis- 
tance below  the  end  of  the  stem  (the  growing  region). 

Annual  stems  increase  in  thickness  until  the  plant  matures. 
This  increase  in  size  is  brought  about  by  the  enlargement  of 
cells  and  by  the  formation  of  additional  cells.  Shrubs  and 
trees  increase  in  thickness  each  growing  season.  This  is  often 
called  secondary  growth  ;  as  we  have  seen,  it  is  brought  about 
by  the  continued  growth  of  the  cambium.  This  layer  of  cells 
produces  new  water-conducting  tissue  and  wood  fibers  on  its 
inner  side,  and  it  produces  food-conducting  tissue  and  bast 
fibers  on  its  outer  side.  As  growth  proceeds  from  year  to 
year,  annual  rings  mark  the  successive  additions  to  the  wood. 
The  bark  also  develops  annual  layers,  but  in  most  woody 
plants  these  are  much  thinner  and  less  conspicuous  than  the 
annual  layers  of  the  wood.  Further,  since  growth  takes  place 
inside  the  cortex,  the  cortex  is  continually  being  split  and 
broken.  The  outer  layers  may  die  and  after  a  few  years  will 
be  gradually  weathered  off.  The  ridges  and  grooves  of  the 
bark  show  how  much  too  small  the  outer  bark  is  to  cover  the 
more  recently  formed  wood.  Smooth,  thin-barked  trees  lose 
their  bark  very  rapidly.  Trees  with  bark  that  is  thick  and 
has  large  ridges  are  the  ones  that  hold  their  bark  more  tena- 
ciously. But  in  all  large  trees  the  bark  contains  only  a  part 
of  the  cortical  layers  that  have  actually  been  formed ;  much 
material  has  scaled  off  and  fallen  away.  It  should  be  noted 
that  as  a  tree  increases  in  diameter,  the  annual  rings  of  wood 


130  Science  of  Plant  Life 

in  the  stem  are  each  year  farther  removed  from  the  corre- 
sponding annual  layers  of  the  bark. 

Annual  monocots  increase  in  thickness  through  the  enlarge- 
ment of  the  bundles  and  by  the  multiplication  and  enlarge- 
ment of  the  pith  cells.  Perennial  monocots,  like  the  bamboo 
and  asparagus,  have  underground  stems  to  which  new  and 
thicker  stem  segments  are  added  each  year.  The  aerial,  erect 
branches  never  increase  in  size  after  they  are  once  mature ; 
but  the  erect  branches  from  old  underground  stems  are  from 
the  beginning  much  thicker  than  those  from  young  plants. 
Consequently,  no  little  bamboo  rod  could  ever  grow  into  a 
bamboo  beam.  No  large  bamboo  beam  was  ever  a  slender 
rod.  These  aerial  branches  come  out  of  the  ground  nearly 
as  thick  as  they  will  be  when  mature.  Asparagus  plants  are 
several  years  old  before  the  underground  stems  send  up  thick, 
upright  branches  suitable  for  marketing. 

Heartwood  and  sapwood.  As  the  trunks  of  trees  increase 
in  thickness,  all  the  living  cells  toward  the  center  of  the  stem 
gradually  die.  The  wood  usually  changes  in  color  after  the 
death  of  these  cells.  In  a  peach  tree  only  the  outer  three  or 
four  annual  rings  may  be  alive.  In  a  walnut  trunk  2  feet  in 
diameter,  all  but  the  outer  2  inches  may  be  dead.  The  dead 
wood  still  helps  to  support  the  enormous  weight  of  the  tree 
top,  but  it  has  nothing  to  do  with  the  conduction  of  water  and 
substances  in  solution.  This  inner  dead  wood  is  called  the 
heartwood;  the  outer  living  wood  is  called  the  sapwood.  The 
heartwood  in  many  species  of  trees  is  much  more  valuable 
than  the  sapwood  for  lumber,  because  of  its  color  and  greater 
durability. 

Stems  in  relation  to  gravity.  The  direction  of  growth  of 
stems  is  for  the  most  part  determined  by  gravity.  The  erect 


The  Structures  and  Processes  of  Stems         131 

type  of  stem  grows  upright  even  in  darkness.  If  these  stems 
are  laid  horizontally,  the  younger  parts  will  grow  faster  on 
the  lower  side  and  the  stem  will  again  become  erect.  This 
response  of  a  plant  organ  to  gravity  is  called  geotropism.1 
If  the  response  is  in  the  direction  of  the  pull  —  that  is,  toward 
the  earth  —  as  in  the  case  of  a  primary  root,  the  organ  is 
said  to  be  positively  geotropic.  If  the  response  is  in  the  op- 
posite direction,  as  in  most  stems,  the  organ  is  said  to  be 
negatively  geotropic.  If  the  response  is  sidewise,  as  in  many 
branches,  the  organ  is  said  to  be  transversely  geotropic. 

Stems  in  relation  to  light.  Light  affects  also  the  direction 
of  growth  of  stems,  as  the  plant  grown  at  a  window  will  show. 
Most  stems  grow  toward  the  strongest  light.  Response  to 
light  is  called  phototropism,  and  most  stems  are  positively 
photo  tropic.  The  stems  of  some  prostrate  plants  are  held 
close  to  the  ground  by  their  response  to  light,  as  is  proved  by 
the  fact  that  if  a  shade  is  placed  over  them,  the  stems  become 
erect.  Some  of  the  common  doorweeds  of  paths  and  waste 
places  are  examples.  There  are  some  prostrate  stems,  of 
course,  that  lie  flat  on  the  ground  because  of  a  lack  of  me- 
chanical tissue  to  hold  them  upright. 

The  direction  of  growth  in  branches  is  a  compromise  be- 
tween the  response  to  light  and  the  response  to  gravity.  In 
some  trees,  like  the  spruce,  the  direction  of  growth  in  the 
branches  is  in  some  way  controlled  by  the  main  stem.  If  the 
top  of  the  main  stem  is  cut  off,  one  or  two  of  the  lateral  branches 
become  negatively  geotropic  instead  of  transversely  geotropic. 

1  The  responses  of  organs  to  external  influences  like  gravity  and  light  are 
called  tropisms  (Greek :  trope,  turning).  "Geotropism"  means  literally  a 
turning  toward  or  away  from  the  earth;  "phototropism,"  a  turning  toward 
or  away  from  light;  "hydrotropism,"  a  turning  toward  or  away  from  water. 


132  Science  of  Plant  Life 

This  means  that  they  will  grow  erect  instead  of  horizontally 
and  will  take  the  place  of  the  main  stem. 

Grafting  and  budding.  In  the  propagation  of  many 
varieties  of  fruit  trees  it  has  been  found  that  seeds  are  not 
satisfactory.  Most  of  our  cultivated  fruit  trees  are  so  highly 
variable  that  their  seedlings  are  not  like  the  parent  plants 
in  quality  of  fruit.  Horticulturists  long  ago  learned  to  over- 
come this  difficulty  by  grafting  a  twig  from  the  desired  variety 
of  tree  on  a  seedling  of  a  similar  tree.  The  graf ted-in  branch 
then  becomes  the  top  of  the  tree,  and  the  fruit  it  bears  is  like 
that  of  the  tree  from  which  it  came. 

In  grafting,  the  plant  that  furnishes  the  root  is  called  the 
stock.  The  twig  that  is  attached  to  it  is  called  the  cion  (Fig. 
76).  In  cleft  grafting,  the  top  of  the  stock  is  cut  off.  The 
stock  is  then  split  and  two  cions  with  chisel-shaped  ends  are 
placed  in  the  cleft,  one  on  either  side,  so  that  the  cambium  of  the 
cion  is  in  close  contact  with  the  cambium  of  the  stock.  The  wound 
is  covered  with  wax  to  prevent  the  drying  out  of  the  tissues. 
If  the  cambium  tissues  are  in  perfect  contact,  they  will  soon 
unite.  New  tissue  will  grow  under  the  wax  and  finally  cover 
the  wound.  If  both  cions  grow,  the  weaker  one  is  removed. 

Whip  grafting  is  the  common  method  of  uniting  cions  to 
small  seedlings.  Usually  this  is  done  at  or  below  the  surface 
of  the  soil.  Both  cion  and  stock  are  cut  obliquely,  and  each 
is  split.  The  upper  half  of  the  oblique  end  of  the  cion  is 
pushed  into  the  cleft  of  the  stock  and  is  bound  firmly  in  pi  ace 
with  raffia  or  twine  (Fig.  76).  Again,  the  success  of  the  graft 
depends  upon  the  contact  between  the  cambium  of  the  cion  and 
the  cambium  of  the  stock. 

In  budding,  a  T-shaped  cut  is  made  on  the  side  of  the  stock, 
through  the  cortex,  down  to  the  cambium.  A  bud  from  a 


The  Structures  and  Processes  of  Stems         133 

tree  of  the  desired  variety,  with  a  small  oval  piece  of  wood 
and  bark  attached,  is  slipped  down  inside  the  cortex  of  the 


FIG.  76.  Methods  of  grafting  and  budding.  At  left,  whip  grafting;  in  middle,  cleft 
grafting;  at  right,  budding.  A  is  the  cion,  and  B  the  stock.  C  shows  the  cion  and 
stock  joined. 

stock  and  tied  firmly  in  place  (Fig.  76).  This  places  the  two 
cambium  layers  in  contact ;  the  two  pieces  unite,  and  the  bud 
develops  into  a  branch.  The  stock  is  then  trimmed,  so  that 
only  the  branch  from  the  cion  bud  remains. 

Grafting  is  commonly  done  in  the  spring ;  budding,  in  the 
early  fall.  The  fruit  produced  on  grafted  or  budded  trees  is 
usually  like  that  of  the  cion,  regardless  of  the  variety  of  stock. 
However,  there  are  cases  in  which  the  cion  is  modified  by  the 
stock.  Discussions  of  these  cases  may  be  found  in  books  on 
horticulture.  Grafting  is  usually  possible  only  between 
closely  related  species  of  plants.  Sometimes,  however,  plants 
that  are  more  remotely  related  may  be  grafted  on  each  other, 
as  for  example  tomato,  tobacco,  potato,  and  nightshade,  or 
the  pear,  apple,  and  quince. 


134 


Science  of  Plant  Life 


The  lifting  of  water  in 

physiology  of  plants  has 


FIG.  77.  Experiment  to  show  the 
lifting  power  of  transpiration  and 
evaporation.  Both  tubes  were  filled 
with  boiled  water  and  placed  in  a  dish 
of  mercury.  In  -C  the  mercury  has 
been  drawn  up  by  transpiration  from 
a  branch  of  arbor- vitae  (A ) ;  in  D,  by 
evaporation  from  a  porous  cup  (B). 

is  pulled  upward  into  the 
Transpiration  is  greatest 


stems.  Nothing  concerning  the 
interested  more  people  than  the 
transport  of  water  from  the  soil 
to  the  topmost  leaves  of  trees. 
Yet  in  spite  of  much  observation 
and  experiment,  the  process  is 
still  only  partially  explained. 

There  can  be  no  doubt  that 
one  of  the  principal  factors  in 
the  rise  of  sap  is  the  evapora- 
tion of  water  from  the  leaves. 
As  the  water  evaporates  from 
the  cells  of  the  mesophyll  in 
transpiration,  water  is  drawn 
from  the  adjoining  water-con- 
ducting tissue  of  the  veins  into 
these  cells  to  take  its  place. 
Water  inclosed  in  tubes  has  a 
high  cohesive  power ;  that  is,  it 
holds  together  like  a  solid.  If  a 
pull  is  exerted  on  the  upper  end 
of  a  column  of  water  in  the 
vessels  of  a  tree,  the  column 
holds  together  like  a  cord  or 
wire,  and  the  whole  column  is 
pulled  upward.  As  the  water 
at  the  upper  end  of  the  water- 
conducting  tissue  moves  into  the 
mesophyll  cells,  additional  water 
blades,  petioles,  and  stems, 
and  the  largest  amounts  of  water 


The  Structures  and  Processes  of  Stems         135 

are  being  lifted  in  trees  during  the  summer.  If  at  this  season 
a  hole  is  bored  into  the  trunk  of  a  tree  and  an  air-tight  con- 
nection made  between  this  hole  and  a  tube  that  has  its  lower 
end  in  a  vessel  of  water,  the  water  is  drawn  into  the  stem,  not 
forced  out.  This  indicates  that  there  is  more  pull  on  the  water 
from  above  than  there  is  pressure  from  below.  It  is  known 
also  that  there  may  be  currents  moving  downward  in  one 
layer  of  the  wood  and  upward  in  another,  although  the  gen- 
eral direction  of  water  transport  is  upward  to  the  leaves.  It 
is  certain  that  the  roots  do  not  force  the  water  up  into  the 
tops  of  trees. 

The  primary  factor,  then,  in  the  rise  of  sap  is  transpiration ; 
the  second  factor  is  the  drawing  of  water  from  the  water- 
conducting  tissue  by  the  mesophyll  cells  to  replace  that  lost 
through  transpiration ;  the  third  factor  is  the  cohesion  of 
water  columns  in  the  long  strands  of  water-conducting  tissue, 
which  makes  it  transmit  the  pull  from  the  mesophyll  cells 
all  the  way  down  to  the  roots.  In  the  chapter  on  roots  we 
shall  learn  how  the  water  passes  from  the  soil  into  the  roots, 
and  to  what  extent  the  roots  aid  in  the  lifting  of  water. 

The  pulling  up  of  water  by  transpiration  is  exemplified 
when  cut  flowers  are  placed  in  a  vase  containing  water.  That 
water  is  drawn  up  into  the  flowers  may  be  shown  by  placing 
the  stems  of  white  flowers  in  water  colored  with  red  ink. 

The  flow  of  maple  sap.  The  water  in  stems  always  con- 
tains a  certain  amount  of  sugar  in  addition  to  mineral  salts. 
In  the  maple,  in  the  early  spring  when  the  days  are  warm  but 
freezing  still  occurs  at  night,  great  quantities  of  sugar  pass 
into  the  water-conducting  tissue.  This  sugar  comes  from  the 
pith  rays  and  other  tissues  where  it  accumulated  in  the  form 
of  starch  during  the  preceding  growing  season.  With  the 


136  Science  of  Plant  Life 

coming  of  warmer  weather  the  starch  is  digested,  and  the  sugar 
formed  from  it  diffuses  into  the  water-conducting  vessels. 

The  earlier  sap  is  the  richest  and  apparently  comes  largely 
from  the  upper  parts  of  the  tree.  The  last  sap  is  more  dilute 
and  probably  comes  from  the  roots.  The  positive  pressure 
that  produces  the  flow  occurs  only  during  the  day ;  at  night 
it  becomes  negative  and  the  sap  flow  ceases.  The  causes  of  the 
pressure  are  only  partly  known.  A  portion  of  it  is  due  to 
the  expansion  of  gas  bubbles  within  the  tree,  but  this  gas 
expansion  accounts  for  only  a  small  part  of  the  pressure. 

Whether  the  flow  shall  continue  for  weeks  or  stop  after  a 
few  days  is  determined  by  weather  conditions ;  but  just  how 
the '  several  weather  factors  like  changes  in  temperature  or 
rainfall  bring  about  the  increase  and  decrease  of  pressure,  is 
unknown.  Even  under  the  most  favorable  conditions  it  is 
not  possible  to  draw  out  of  a  tree  more  than  5  per  cent  of  the 
food  that  it  contains. 

A  flow  of  sap  somewhat  similar  to  that  in  the  maples  occurs 
in  many  species  of  trees,  as  in  the  birch,  butternut,  and 
hornbeam.  Sugar  and  sirup  are  made  from  the  hard  maples. 

PROBLEMS 

1.  A  tree  increases  in  height  at  the  rate  of  2  feet  a  year.     When  the  tree  is 
5  years  old,  a  nail  is  driven  into  the  trunk  2  feet  from  the  ground.     How  far 
from  the  ground  will  the  nail  be  when  the  tree  is   10  years  old? 

2.  Why  is  the  timber  from  monocot  stems  less  useful  for  building  purposes 
than  that  from  dicot  stems? 

3.  What  are  the  woods  principally  used  in  your  locality  for  interior  finishing 
and  for  making  shingles,  posts,  and  flooring?     What  properties  do  these 
woods  have  that  fit  them  for  these  special  uses? 

4.  What  kinds  of  wood  are  used  in  the  making  of  furniture ?     Why? 

5.  How  is  a  log  quarter-sawed?     Why  is  quarter-sawed  lumber  more  valuable 
than  that  cut  otherwise? 


The  Structures  and  Processes  of  Stems         137 

6.  Why  is  lumber  seasoned  by  piling  it  in  lumber  yards  for  months  or  years 
before  it  is  used  ? 

7.  What  is  charcoal?     How  is  it  manufactured?     What  are  its  uses? 

8.  Why  is  charcoal  made  from  peach  and  apricot  pits  and  the  shells  of  nuts  pre- 
ferred as  an  absorbent  in  gas  masks? 

9.  What  commercial  products  are  made  by  the  distillation  of  wood? 

10.   What  substances  are  used  to  preserve  railroad  ties  and  posts  from  decay  ? 
From  what  are  these  substances  derived? 


Suggestions  for  Field  Work  to  Precede  Chapter  Twelve 

1.  A  field  trip  to  a  greenhouse  will  help  to  make  clear  the  im- 
portant factors  of   the  environment.    Observe  how  the  water, 
light,  temperature,  soil,  animal,  and  plant  factors  are  taken  care 
of.     (Animal  factors:  scale,  aphids,  mealy  bugs,  and  white  flies. 
Plant   factors:   overcrowding,   bacterial  and  fungous  diseases.) 

2.  In  the  field,  compare  the  environmental  factors  in  a  lake  or 
swamp  with  those  on  a  hilltop  or  slope.     What  are  the  notable 
effects  of  these  factors  on  leaves,  stems,  and  roots?     Write  the 
results  of  your  observations  in  the  form  of  a  table. 

3.  Examine  trees  growing  in  the  open  to  see  how  the  wind 
affects  the  shape  of  the  crown.     Is  the  trunk  in  the  center  of  the 
crown?     What  is  the  direction  of  the  prevailing  wind  in  your 
locality?     In  what  direction  are  the  branches  of  trees  generally 
longest  in  your  locality  ?     Shortest  ?     What  is  the  relation  of  this 
branch  development  to  the  direction  of  the  prevailing  wind  ? 

4.  Field  trip  to  a  dense  wood.     Study  (i)  the  environmental 
conditions  of  the  large  trees;  (2)  of  the  young  trees;  (3)  of  the 
climbing  plants ;  (4)  of  the  low  herbs ;  (5)  of  the  mosses,  lichens, 
or  ferns  on  the  tree  trunks ;  and  (6)  of  the  mosses,  lichens,  or  ferns 
growing  on  the  soil.     Write  your  notes  in  the  form  of  a  table  show- 
ing relative  amounts  of  light,  water,  and  mineral  salts  available 
to  plants  belonging  to  each  of  the  above  groups,  and  relative  dry- 
ness  of  air  to  which  they  are  exposed. 


138 


CHAPTER   TWELVE 

THE   ENVIRONMENT    OF   PLANTS 

EVEN  the  most  casual  observation  suggests  that  plants  are 
greatly  affected  by  the  conditions  under  which  they  grow. 
The  grasslands,  the  forests,  and  the  deserts  of  North  America 
are  occupied  by  different  plants  because  of  the  difference  of 
conditions  in  these  areas.  The  white  oak  growing  in  the  open 
has  broad,  spreading  branches  and  a  large  crown,  while  in  the 
forest  it  is  more  columnar  and  has  a  crown  at  the  upper  level 
of  the  forest.  The  sugar  maple  becomes  a  great  tree  or  re- 
mains a  mere  shrub  according  to  whether  it  grows  in  rich  soil 
or  in  a  crevice  of  a  rock.  The  internal  structure  of  a  plant 
is  also  modified  by  the  external  conditions  under  which  it 
grows ;  the  leaves  of  many  plants  which  are  thin  and  tender 
when  there  is  an  abundant  water  supply,  become  thick  and 
leathery  when  grown  under  conditions  of  drought.  Since 
conditions  of  growth  show  so  great  an  effect  upon  the  form, 
size,  structure,  abundance,  and  distribution  of  plants,  we  should 
know  the  principal  factors  that  make  up  a  plant's  environ- 
ment before  going  further  in  the  study  of  the  plant  itself. 
It  is  especially  important  to  understand  these  factors,  because 
the  purpose  of  a  great  part  of  agricultural  practice  is  to 
modify  the  environment  of  the  plants  that  are  being  grown. 

Definition  of  environment.  By  the  environment  of  a  plant 
is  meant  the  complex  of  all  those  influences  outside  the  plant 
which  directly  or  indirectly  affect  its  physiological  processes, 
its  structures,  and  its  development  and  propagation.  These 
influences  are  numerous  and  are  usually  spoken  of  as  factors. 
The  natural  habitat  of  a  plant  is  a  combination  of  environ- 
mental factors  favorable  to  the  complete  development  of  the 
plant.  The  factors  include  the  chemical  and  physical  prop- 

139 


140 


Science  of  Plant  Life 


erties  of  the  soil  and  the  air  surrounding  the  plant;  also 
light,  gravity,  and  the  influences  of  animals  and  other  plants 
(Fig.  78). 


FIG.  78.     Diagram  illustrating  the  environment  of  land  plants. 

The  soil  factors.  If  a  plant  is  to  grow  in  a  soil,  the  soil 
must  furnish  it  with  sufficient  water  for  transpiration  and  the 
manufacture  of  food.  At  the  same  time,  the  soil  must  not  be 


The  Environment  of  Plants 


141 


Irrigation  Division,  U.  S.  Dept.  oj  Agriculture 

FIG.  79.     Irrigated  and  unirrigated  sugar  cane,  showing  the  value  of  sufficient 
water  in  the  growing  of  this  crop. 

so  filled  with  water  as  to  exclude  air  from  the  roots.  In  order 
that  the  roots  may  penetrate  the  soil  readily,  it  should  not  be 
too  resistant ;  but  it  should  be  compact  enough  to  afford  the 
plant  a  firm  anchorage. 

The  soil  must  supply  also  certain  indispensable  chemical 
elements  used  by  plants  in  the  manufacture  of  food. 
These  elements  are  potassium,  calcium,  magnesium,  phos- 
phorus, nitrogen,  iron,  and  sulfur.  In  soils  that  contain 
insufficient  amounts  of  any  of  these  substances  the  growth  of 
plants  is  hindered,  and  certain  plants  are  excluded  from  such 
a  soil.  One  of  the  purposes  in  using  chemical  fertilizers  is 
to  add  to  soils  or  to  liberate  in  them  sufficient  quantities  of 
all  the  elements  essential  to  the  vigorous  growth  of  crop 
plants.  It  should  be  remembered  that  in  addition  to  the 


142 


Science  of  Plant  Life 


seven  elements  mentioned  above,  plants  use,  for  the  manu- 
facture of  food,  carbon,  hydrogen,  and  oxygen,  which  they 
derive  from  water  and  carbon  dioxid. 

Most  plants  grow  best  when  the  soil  is  neutral  or  slightly 
alkaline.  Red  clover,  alfalfa,  and  blue  grass,  for  example, 
cannot  withstand  an  acid  soil.  By  the  addition  of  lime,  acid 
soils  may  be  neutralized  or  made  alkaline.  This  explains 
the  common  practice  of  putting  lime  or  wood  ashes  on  lawns 
where  a  growth  of  blue  grass  is  desired.  Some  plants  are 
favored  by  an  acid  soil.  Cranberries,  blueberries,  and  redtop 
grass  are  examples  of  such  plants.  In  arid  regions,  the 
evaporation  of  water  may  cause  salts  to  accumulate  in,  the 
surface  layers  of  the  soil  to  such  an  extent  that  most  or  all 
plants  are  excluded. 

Another  soil  factor  of  great  importance  is  humus.     This 


U.  S.  Dept.  of  Agriculture 

FIG.  80.    A  Kansas  cornfield.    The  soil  is  rich  in  humus,  and  the  plants  attain  a 
height  of  12  to  14  feet. 


The  Environment  of  Plants 


143 


FIG.  81.     The  results  of  deep  tillage  and  shallow  tillage. 

material,  which  gives  the  brown  and  black  colors  to  rich 
agricultural  land,  is  composed  of  the  partially  decayed  re- 
mains of  plants.  Leaves  and  other  plant  organs  that  fall 
to  the  ground  are  slowly  changed  and  broken  up  by  bacteria 
and  other  agencies  until  only  the  brown,  powdery  humus 
remains. 

Humus  favors  plant  growth  by  increasing  the  water-hold- 
ing capacity  of  the  soil  and  so  rendering  the  water  supply 
more  uniform  throughout  the  growing  season.  It  improves 
the  physical  properties  of  the  soil  by  making  it  mellow.  Hu- 
mus also  makes  it  possible  for  bacteria  and  other  organisms 
that  increase  fertility  to  live  within  the  soil. 

In  agricultural  literature  the  importance  of  the  soil  factors 
is  emphasized.  Plowing,  harrowing,  disking,  and  cultivating 
are  methods  of  keeping  the  soil  in  good  physical  condition. 
Adding  manures  to  increase  the  humus,  and  adding  nitrates, 
phosphates,  and  other  salts  as  fertilizers,  are  methods  of  im- 


144  Science  of  Plant  Life 

proving  the  chemical  composition  of  the  soil  and  to  a  less 
extent  its  physical  condition.  Irrigating  the  land  where  the 
water  supply  is  ina4equate,  and  under  draining  it  where  the 
soil  moisture  is  excessive,  are  further  means  of  improving  soil 
conditions  for  the  growth  of  crops.  The  soil  requires  these 
special  attentions  for  the  growth  of  domesticated  plants  be- 
cause they  must  not  only  live,  as  do  wild  plants,  but  they 
must,  in  one  form  or  another,  yield  a  profitable  return. 

Atmospheric  water.  The  water  in  the  air  affects  plants 
directly  in  several  ways.  The  moistness  or  dry  ness  of  the 
air  determines  whether  less  water  or  more  is  required  for 
transpiration,  and  the  amount  of  water  precipitated  from  the 
air  in  the  form  of  rain  determines  to  a  large  extent  the  amount 
of  water  available  in  the  soil.  Atmospheric  water  conderised 
in  the  form  of  fog  and  cloud  reduces  transpiration  and  also 
lessens  the  amount  of  light  that  reaches  the  plant. 

The  distribution  of  rainfall  through  the  year  is  of  the  great- 
est importance  to  vegetation.  When  the  period  of  heaviest 
rainfall  coincides  with  the  hottest  part  of  the  year,  the  con- 
ditions are  best  for  the  rapid  growth  of  plants.  If  the  rain- 


FIG.  82.  Cross  sections  of  kernels  of  hard  or  macaroni  wheat.  This  wheat  is  grown  in 
dry  regions  and  is  valued  because  of  its  large  content  of  protein.  In  the  figures  the 
flinty  or  high  protein  parts  are  shaded  and  the  soft  or  starchy  parts  are  white.  When 
the  wheat  is  grown  under  the  conditions  of  dry  farming,  the  protein  content  is  highest 
04);  when  regularly  irrigated,  the  same  wheat  produces  soft,  starchy  grains  (C).  An 
intermediate  condition  is  shown  by  B.  This  exemplifies  the  effect  of  the  water  balance 
on  the  composition  of  a  grain. 


The  Environment  of  Plants 


145 


146  Science  of  Plant  Life 

fall  is  scanty  during  the  time  of  highest  temperatures,  plants 
are  hindered  in  their  growth,  and  only  xerophytes  may  be 
able  to  withstand  the  conditions. 

The  temperature  factor.  As  one  goes  north  or  south  from 
the  equator,  the  temperatures  of  the  soil  and  air  decrease. 
Increasing  altitude  in  mountains  brings  about  the  same  effects. 
Temperature  directly  influences  the  rate  of  all  plant  pro- 
cesses, and  most  plants  grow  best  under  certain  rather  fixed 
temperature  conditions.  For  tropical  plants,  air  temperatures 
above  90  degrees  F.  are  most  favorable.  Temperate  plants  de- 
velop best  at  between  60  and  90  degrees  F.  Arctic  and  alpine 
plants  grow  at  temperatures  but  little  above  the  freezing  point. 

Air  temperatures  are  greatly  influenced  by  air  drainage. 
Cold  air  is  heavier  than  warm  air ;  consequently  it  accumulates 
in  low  grounds  and  reduces  the  temperature  there.  Frost 
occurs  later  in  the  spring  and  earlier  in  the  autumn  in  low 
places  than  on  hills.  Crop  plants  like  beans,  that  are  easily 
injured  by  frost,  can  be  planted  earlier  and  grown  later  on 
uplands.  Peach  orchards  are  more  profitable  on  uplands 
than  in  valley  bottoms,  because  on  the  uplands  they  are  more 
likely  to  escape  late  spring  frosts. 

The  time  during  which  the  temperature  remains  above  the 
freezing  point  is  the  growing  season.  In  the  tropics  this  ex- 
tends throughout  the  year.  In  arctic  and  alpine  regions  it 
may  be  reduced  to  2  or  3  months.  The  temperature  of  the 
air  and  the  length  of  the  growing  season  determine  the  amount 
of  food  a  plant  may  manufacture,  and  consequently  the 
amount  of  growth. 

Soil  temperatures  also  are  important.  Dark-colored  soils 
are  warmer  than  light-colored  soils,  because  they  absorb  the 
sun's  rays  more  readily.  Well-drained  soils  are  wanner  than 


The  Environment  of  Plants 


DATE  WHEN  HARVEST  OF  SPRING  OATS  BEGI] 
A0e.ii 


MAY 


FiG.  84.     Map  showing  date  of  oat  harvest  in  different  parts  of  the  United  States ; 
an  example  of  the  effect  of  temperature  on  the  maturing  of  plants. 

wet  soils,  (i)  because  less  heat  is  required  to  raise  their  tem- 
peratures, and  (2)  because  the  temperature  of  a  wet  soil  is 
lowered  by  the  constant  evaporation  of  water.  The  most 
valuable  farm  lands  are  those  with  dark-colored,  well-drained 
soils.  On  north  slopes,  soils  do  not  warm  up  so  rapidly  in  the 
spring,  and  plants  growing  there  start  their  growth. later  than 
do  those  on  the  south  slopes  of  the  same  hills.  Peach  growers 
prefer  not  only  uplands  but  north  slopes.  Why? 


148 


Science  of  Plant  Life 


FIG.  85.    Effects  of  wind  on  trees  along  the  Bay  of  Fundy,  Nova  Scotia. 

The  light.  The  amount  of  light  available  to  a  plant  de- 
pends primarily  upon  the  intensity  of  the  sunshine.  This  is 
greatest  in  the  tropics  and  least  at  the  poles.  The  total 
amount  of  light  is  influenced  also  by  the  length  of  the  day. 
At  the  equator  the  daylight  lasts  12  hours;  at  the  pole, 
the  light  continues  all  summer.  So  tropical  plants  have  in- 
tense light  during  half  of  each  day,  while  arctic  plants  have 
weak  light  continuously  through  the  growing  season. 

Locally  the  light  is  modified  by  clouds  and  fogs.  These  are 
much  more  prevalent  along  the  seacoast  than  inland,  and  are 
particularly  common  along  the  Pacific  coast  from  Alaska  to 
California.  The  slope  of  the  land,  especially  in  mountain 
regions,  may  increase  or  decrease  the  intensity  and  the  length 
of  daylight.  Finally,  plants  may  have  their  light  reduced  or 
cut  off  by  trees  or  other  objects. 


The  Environment  of  Plants 


149 


Gravity.  Gravity  is  an  im- 
portant environmental  factor, 
largely  because  of  its  influence 
on  the  direction  of  growth  in 
stems,  roots,  and  other  organs 
(page  131).  Light  influences 
also  the  growth  of  the  various 
parts  of  the  plant.  Conse- 
quently the  position  of  the 
aerial  organs  of  plants  is  to  a 
large  extent  determined  by  the 
combined  influences  of  light 
and  gravity. 

Wind.  Winds  and  air  cur- 
rents are  of  importance,  as 
they  affect  the  rate  of  tran- 
spiration or  modify  the  tem- 
perature. It  may  take  10 
minutes  for  your  wet  hands  to 
dry  in  still  air,  but  if  you  hold 

them  before  an  electric  fan  they  will  be  dry  in  2  or  3  minutes. 
The  passing  of  an  increased  volume  of  air  over  a  wet  surface 
increases  evaporation,  and  wind  affects  the  transpiration  of 
plants  in  the  same  way.  In  drying  your  hands  before  a  fan, 
notice  also  the  cooling  effect  of  the  breeze.  Leaves  are  cooled 
by  transpiration  in  the  same  way.  Winds  also  may  cause 
important  modifications  in  the  forms  of  plants,  and  occasion- 
ally violent  winds  may  destroy  large  areas  of  timber  and 
crops. 

Animals.     Leaf-eating  insects,  such  as  the  potato  beetle, 
injure  the  plant  by  destroying  the  food-making  apparatus. 


W.  S.  Cooper 

FIG.  86.  Western  white  pine  on  Long's 
Peak,  Colorado,  showing  the  effects  of 
violent  winds  and  of  wind-driven  snow. 


150  Science  of  Plant  Life 

It  is  estimated  that  grasshoppers  and  leaf-hoppers  often  eat 
as  much  of  the  grass  in  a  pasture  as  do  the  farm  animals. 
Plant  lice  and  scale  insects  remove  the  sap  from  the  cells  of 
the  tender  growing  parts  and  may  kill  the  entire  plant.  Other 
animals,  like  the  earthworm,  favor  the  growth  of  plants  by 
loosening  the  soil  and  promoting  the  change  of  fallen  leaves 
to  humus.  Herbivorous  (Latin :  herba,  herb,  and  vorare,  to 
eat)  wild  animals,  like  the  rabbits,  squirrels,  and  deer,  markedly 
affect  natural  vegetation,  while  the  domesticated  cattle,  sheep, 
and  hogs  to  a  large  extent  determine  what  plants  can  survive 
in  pastures  and  grazing  lands.  Man,  more  than  all  other 
animals  put  together,  has  modified  the  natural  vegetation  of 
the  earth.  In  some  cases  he  has  destroyed  it ;  in  other  cases 
he  has  encouraged  and  protected  it.  Most  of  all,  he  has 
selected  certain  plants  and  made  of  them  the  food  supply  of 
the  world. 

Other  plants  as  an  environmental  factor.  Other  plants, 
such  as  weeds  growing  among  cultivated  crops,  may  modify 
the  environment  of  plants  by  shading  them,  by  removing  water 
from  the  soil,  and  possibly  by  producing  poisonous  substances 
in  the  soil.  Or  a  plant  may  directly  affect  another  plant 
by  growing  on  it  and  taking  its  nourishment  from  it.  For 
example,  the  mistletoe  grows  on  trees  and  injures  them.  Corn 
smut  and  wheat  rust  live  on  corn  and  wheat  and  decrease 
or  prevent  the  production  of  grain. 

The  complexity  of  the  environment.  The  environment  of 
plants  is  made  up  of  many  factors,  and  the  factors  them- 
selves are  more  or  less  dependent  upon  each  other.  Conse- 
quently it  is  often  difficult  to  determine  definitely  the  cause 
of  a  particular  effect  that  is  undoubtedly  produced  by  some- 
thing in  the  environment.  But  as  our  knowledge  of  botany 


The  Environment  of  Plants  151 

advances,  we  are  able  to  relate  more  and  more  of  the  effects 
that  we  observe  in  plants  to  definite  factors  in  their  environ- 
ment. The  farmer,  gardener,  or  forester  is  often  able  to  use 
this  knowledge  in  making  the  environment  more  favorable  for 
the  plants  which  he  grows. 

Subdivisions  of  botany.  Three  of  the  great  subdivisions  of 
botany  are  morphology,  physiology,  and  ecology.  The  study 
of  the  structures  of  plants  is  plant  morphology  (Greek  morphos, 
form,  and  logos,  study  of).  The  study  of  the  processes  of 
plants  such  as  transpiration  and  photosynthesis  is  plant 
physiology  (Greek:  physis,  nature,  and  logos,  study).  The 
study  of  plant  structures  and  processes  in  relation  to  the  en- 
vironment, or  as  they  are  modified  by  the  factors  that  make  up 
the  environment,  is  called  ecology  (Greek :  oikos,  home,  and 
logos],  or  ecological  botany.  Since  the  environment  deter- 
mines what  plants  can  live  in  a  particular  place,  ecology  in- 
cludes also  the  study  of  the  distribution  of  plants  on  the 
earth's  surface  and  attempts  to  account  for  that  distribution. 
For  this  reason  the  environmental  factors  that  affect  plants 
are  also  spoken  of  as  ecological  factors ;  and  the  changes  in 
structure  that  adjust  plants  to  particular  habitats  may  be 
called  ecological  modifications. 

PROBLEMS 

1.  What  is  the  principal  factor  that  limits  the  growing  of  oranges  in  this  coun- 
try to  Florida  and  southern  California? 

2.  Why  are  beans  planted  a  month  later  than  peas? 

3.  What  environmental  factor  determines  that  melons  and  squashes  should 
not  be  planted  until  late  spring? 

4.  What  factors  in  the  environment  make  it  necessary  to  spray  potatoes  ? 

5.  What  is  the  principal  factor  limiting  the  production  of  crops  on  the  Western 
plains? 

6.  Why  are  plants  absent  from  the  larger  sand  dunes? 


152  Science  of  Plant  Life 

7.  Why  are  plants  absent  from  sea  beaches  and  from  the  beaches  of  large 
lakes? 

8.  Why  do  trees  along  streams  have  their  longest  branches  out  over  the  water  ? 

9.  Why  do  trees  along  streams  frequently  lean  outward  over  the  water? 

10.  Why  do  trees  on  steep  hillsides  have  their  shortest  branches  toward  the 
hill? 

11.  What  are  the  most  favorable  environmental  conditions  for  the  growing  of 
rice,  celery,  onions,  dates,  bananas,  sugar  cane,  guayule,  sisal,  tulips,  and 
pecan  trees? 

12.  How  do  the  plants  and  fruits  of  blackberries  and  dewberries  grown  in 
bottom  lands  differ  from  those  grown  on  dry  hillsides?     Why? 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Thirteen 

1.  Sketch  a  few  twining  stems.     Note  the  twist  of  the  stem  as 
shown  by  surface  markings. 

2.  Sketch  examples  of  tendrils.     The  Boston  ivy,  gourd,  pea, 
sweet  pea,  and  grape  are  readily  obtainable.     Is  the  spiral  con-1 
tinuously  coiled  in  one  direction  ?     Why  ? 

3.  Dissect  a  cactus  stem.     Note  the  nodes,   internal  water- 
storage  and  food-storage  tissue,  and  green  tissue. 

4.  Make  drawings  of  a  rootstock,  corm,  bulb,  and  tuber.  Divide 
each  of  them  into  halves  by  cutting  through  the  longitudinal  axis. 
Compare  their  advantages  as  organs  of  propagation  and  storage. 

5.  Examine  stems  of  water  hyacinth,  bulrush,  and  pondweed, 
noting  air  chambers. 

6.  Examine  a  cross  section  of  a  grapevine  or  a  rattan  stem. 
Note  the  large  size  of  the  water  tubes. 


CHAPTER   THIRTEEN 

ECOLOGICAL   GROUPS   OF   STEMS 

IN  the  tenth  chapter  we  discussed  the  normal  structures  of 
the  upright  stems  of  land  plants.  It  was  pointed  out  that 
each  of  the  three  great  groups  of  seed  plants  has  a  charac- 
teristic arrangement  of  its  tissue  systems.  The  essential 
tissues,  like  the  water-conducting,  food-conducting,  storage, 
and  mechanical  tissues,  are  present  in  all. 

Stem  structures  and  habitats.  Plants  growing  in  different 
habitats,  as  ponds,  swamps,  and  deserts,  have  very  different 
stem  structures.  The  stem  of  a  leafless  desert  plant  must  of 
necessity  be  different  from  that  of  a  leafy  submerged  plant. 
The  tropical  climber  has  a  stem  quite  unlike  that  of  a  plant 
whose  main  stem  is  underground.  These  differences  con- 
sist not  so  much  in  the  arrangement  of  the  several  tissues  as 
in  modifications  in  their  amounts  and  proportions. 

Stems  of  mesophytes.  The  native  plants  of  the  eastern 
United  States  grow  under  medium  conditions  of  moisture, 
light,  and  temperature ;  and  they  are  characterized  by  large 
leaf  area,  leaves  of  soft  texture,  and  much-branched  stems. 
The  vegetation  culminates  in  the  forests  of  the  rich,  well- 
watered  soils  of  the  river  valleys.  Here  may  be  found  oaks, 
walnuts,  elms,  and  sycamores  from  100  to  150  feet  in  height 
and  with  trunks  from  4  to  14  feet  in  diameter. 

In  the  moist  canons  of  the  Sierras  of  California,  the  giant 
sequoia  reaches  heights  of  from  250  to  320  feet  above  the 
ground,  with  extreme  trunk  diameters  of  35  feet.  This  tree 
is  the  largest  and  is  perhaps  the  oldest  of  all  living  things. 
The  redwood,  its  near  relative,  grows  in  the  fog-abounding 
ravines  of  the  Coast  Ranges.  Its  trunk  does  not  attain  a 
diameter  of  more  than  28  feet,  but  it  surpasses  the  giant 

154 


Ecological  Groups  of  Stems 


I5S 


sequoia  in  height.  Really  to  appreciate  the  size  of  these  trees 
you  should  pace  off  a  distance  equal  to  the  diameter  of  the 
trunk  and  calculate  how  many  times  the  height  of  your  school 
building  a  giant  sequoia  is.  Then  try  to  imagine  how  one 
of  the  Big  Trees  would  look  if  it  stood  in  your  school  yard. 


Stems  of  climbing  plants. 

Among  mesophytes  are 
many  vines  with  exceed- 
ingly long,  slender  stems. 
The  Virginia  creeper,  wild 
cucumber,  and  grape  have 
stems  from  50  to  300  feet 
in  length.  These  long  stems 
enable  them  to  spread  their 
leaves  over  the  tops  of  large 
trees .  Climbers  may  attach 
themselves  to  their  support 
FIG.  87.  Tendrils  of  wild  cucumber.  Note  by  twining  about  it  by  ten- 

the  coiling  of  the  tendril  by  which  the  plant  is  drils   Or  bv  SUDDOrtive  TOOtS. 

drawn  nearer  the  support,  and  the  reversal  of  ^        ,   .,  .    , .       , 

the  spiral  in  different  parts  of  the  tendril.  Tendrils  are  Specialized   <M> 

Is  there  always  a  reversal  in  coiled  tendrils?  gans   developed   in   place  of 


156 


Science  of  Plant  Life 


I  branches  or  leaves.  They  re- 
spond to  contact  with  a  sup- 
port by  coiling  tightly  about  it. 
After  attaching  themselves, 
they  develop  mechanical  tissue 
which  gives  the  plant  a  firmer 
support.  In  some  vines,  like 
the  Boston  ivy,  the  tendrils 
have  at  their  tips  sensitive  disks 
which  become  cemented  to  the 
support.  This  type  of  tendril  is 
especially  effective  in  taking 
hold  of  the  bark  of  trees,  rock 
cliffs,  and  walls  (Fig.  87). 

In  the  tropics,  climbing  stems 
may  attain  a  length  of  more 
than  looofeet.  Thus  the  water 
transpired  by  the  terminal 
leaves  has  to  be  carried  for 
about  a  fifth  of  a  mile  within 
the  plant.  This  suggests  the 

need  of  an  efficient  conductive  system  in  a  climbing  plant,  and 
explains  why  the  bulk  of  its  slender  stem  is  made  up  of  con- 
ductive tissue. 

Stems  of  hydrophytes.  We  have  seen  that  submerged 
leaves  have  distinctive  forms  and  characteristic  internal 
structures.  Under- water  stems  also  differ  from  those  of 
land  plants.  In  floating  plants  like  duckweed,  water  hya- 
cinth, and  Salvinia,  the  stems  are  short.  Their  conductive 
systems  are  poorly  developed,  and  they  are  practically  without 
mechanical  tissue. 


FIG.  88.    Climbing  stems  on  a  tree 
trunk  in  a  tropical  forest. 


Ecological  Groups  of  Stems  157 

Other  hydrophytes,  like  the  pond  weeds  and  water  lilies, 
are  rooted  in  the  soil,  and  their  stems  bear  submerged  or 
floating  leaves.  The  stems  have  little  or  no  mechanical  tis- 
sue. As  compared  with  land  plants,  the  conductive  system 
is  much  reduced.  Many  of  these  hydrophytes  develop 
underground  rootstocks  and  tubers.  For  this  reason  the 
plants  commonly  grow  in  masses. 

A  third  group  of  hydrophytes  are  those  like  the  cat-tails, 
rushes,  bulrushes,  and  sedges,  whose  roots  and  stem  bases 
may  be  under  water,  while  the  upper  parts  are  exposed  to  the 
air.  These  plants  have  both  the  conductive  and  mechanical 
tissues  well  developed.  This  is  in  keeping  with  the  fact  that 
such  plants  are  exposed  to  the  action  of  wind  and  wave  and 
to  the  conditions  that  bring  about  normal  transpiration. 

The  most  distinctive  feature  of  submerged  stems  is  the 
presence  of  large  air  chambers  extending  throughout  their 
length.  When  the  stems  are  broken  open,  the  tissues  are 
seen  to  occupy  much  less  space  than  the  air  cavities.  We  may 
properly  speak  of  "  intercellular  spaces  "  inmesophytic  stems ; 
in  describing  hydrophytes,  the  term  "  air  cavities  "  is  more 
appropriate.  They  buoy  up  the  plant  and  provide  an  internal 
atmosphere  for  gas  exchanges  between  the  leaves  and  roots. 

Stems  of  xerophytes.  The  xerophytes  are  the  character- 
istic plants  of  deserts  and  dry  plains.  They  occupy  sand 
dunes  and  sand  plains  along  the  Atlantic  coast  and  on  the 
shores  of  the  Great  Lakes.  They  may  be  found  locally  on 
rock  cliffs  and  on  dry,  exposed  hilltops,  situations  in  which  a 
reduced  water  supply  in  the  soil  is  accompanied  by  atmos- 
pheric conditions  that  promote  rapid  transpiration  and  in 
which  the  plants  are  periodically  or  continuously  subjected 
to  drought.  Plants  that  thrive  in  these  habitats  show  a  re- 


Science  of  Plant  Life 


FIG.  89.    A  desert  scene  in  Arizona. 


Caspar  W.  Hodgson 


duced  leaf  area  or  a  complete  absence  of  leaves.     The  stems 
also  may  be  reduced  in  size  and  amount  of  branching. 

The  cactuses  represent  extreme  examples  of  this  type  of 
plant.  Leaves  are  wanting ;  and  the  stems  are  columnar, 
often  ridged  and  fluted,  and  always  thick  and  fleshy.  The 
photosynthetic  work  in  cactuses  is  done  by  the  cortical  tis- 
sue. As  the  green  surface  is  small  compared  with  the  green 
surface  in  mesophytic  plants,  food  manufacture  is  slower 
and  growth  is  correspondingly  less.  Some  of  the  cactuses  of 
Mexico  attain  heights  of  60  feet.  The  cactus  form  points 
clearly  to  one  of  the  most  characteristic  features  of  desert 
plants ;  namely,  water  storage.  A  single  plant  may  contain 
from  15  to  20  gallons  of  water.  As  the  plant  loses  moisture 
so  slowly,  it  may  continue  to  live  for  several  years  without 
an  additional  supply  of  water.  Other  desert  plants  that 
accumulate  water  in  the  leaves  were  mentioned  on  page  58. 


Ecological  Groups  of  Stems  159 

Underground  stems.  Many  plants  belonging  to  each  of  the 
three  ecological  groups,  xerophytes,  mesophytes,  and  hydro- 
phytes, possess  underground  stems.  Underground  stems  are 
particularly  useful  as  storage  places  for  accumulated  food  and 
water,  and  as  organs  for  propagating  the  plant  (page  217). 

The  commonest  type  of  underground  stem  is  the  rootstock. 
Rootstocks  are  horizontally  growing  stems,  from  which  the 
aerial  stems  arise.  They  may  be  slender,  or  thick  and  fleshy. 
Usually  they  have  small  scale  leaves  and  buds  at  the  nodes, 
and  roots  that  arise  from  the  nodes  or  from  the  entire  under 
surface.  The  presence  of  nodes  is  the  external  feature  of 
underground  stems  that  distinguishes  them  from  roots. 

In  many  of  the  grasses  and  grasslike  plants  rootstocks  de- 
velop rapidly  in  all  directions,  sending  up  erect  branches  at 
short  intervals.  The  rootstocks  and  their  accompanying 
roots  soon  become  mixed  with  those  of  adjoining  plants, 
finally  forming  a  closely  interwoven  mat  which  is  the  "  turf  " 
of  lawns  and  meadows.  Turf -forming  grasses  are  often  of 
great  value  for  holding  in  place  the  soil  of  embankments, 
dikes,  and  levees.  In  these  plants  the  rootstocks  are  mainly 
useful  in  spreading  or  extending  the  plant.  Bermuda  grass 
and  Johnson  grass  are  sometimes  troublesome  weeds  because 
of  their  extensive  rootstock  system.  The  sand-reed  grass 
has  been  planted  extensively  in  Europe  and  in  America  to 
hold  drifting  sand  in  place,  and  to  prevent  the  sand  from  in- 
vading towns  and  cultivated  fields. 

In  plants  like  the  May  apple,  Solomon's  seal,  and  yellow 
water  lily,  the  rootstock  not  only  causes  the  plant  to  spread, 
but  it  also  accumulates  a  part  of  the  food  manufactured  each 
season  and  thus  serves  as  a  storage  organ.  It  is  this  store  of 
food  and  the  readiness  with  which  the  rootstock  sends  up 


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Science  of  Plant  Life 


shoots,  that  make  the  bindweed  so  difficult  to  eradicate 
from  cultivated  fields,  gardens,  and  hedgerows. 

A  short,  upright,  fleshy 
rootstock,  like  that  of  the 
jack-in-the-pulpit,  Cala- 
dium  (elephant's  ear),  or 
gladiolus,  is  called  a  corm. 
Corms  contain  large 
amounts  of  food,  and  by 
the  development  of  their 
lateral  buds  may  serve 
to  reproduce  the  plant  as 
well  as  to  carry  it  over 
the  winter.  The  dasheen, 
a  tropical  plant  which 
resembles  the  Caladium, 
and  which  has  recently 
been  introduced  into  the 

FIG.  90.    Dasheen  and  edible  conns  produced  United      States,      has      an 

by  it.     The  dasheen  is  related  to  the  common  ^^     QQTm     ^     ^     ^ 

elephant  s  ear    or  Calad^um,  and  is  extensively 

grown  in  the  tropics  for  food.    In  the  states  important  SOUrce  of  food. 

along  the  Gulf  Coast  it  is  being  introduced  as  a  A  bulb  ig  ft  flesny  Un- 
food  plant. 

derground  bud,  made  up 

of  a  short  stem  covered  with  several  layers  of  thick  scales  in 
which  food  is  stored.  Tulips,  hyacinths,  and  onions  are 
commonly  propagated  by  means  of  bulbs. 

By  planting  bulbs  of  the  tulip  in  autumn,  we  can  have 
flowers  early  in  the  following  spring,  whereas  if  we  planted  the 
seeds,  we  should  have  to  wait  several  years  for  flowers.  Fur- 
thermore, tulips  do  not  grow  well  except  in  a  very  moist 
climate,  and  the  development  of  large,  vigorous  bulbs  is  im- 


Ecological  Groups  of  Stems 


161 


FIG.  91.     Amaryllis  bulb. 


possible  in  most  parts  of  the  United  States.  For  this  reason 
nearly  all  our  tulip  bulbs  are  brought  from  Holland.  The 
importation  of  bulbs  from  countries 
where  they  grow  particularly  well  is 
an  important  industry  and  enables 
us  to  have  many  flowers  which  can- 
not be  so  successfully  propagated  in 
our  climate. 

Tubers  are  the  enormously  thick- 
ened ends  of  short  underground 
stems.  The  potato,  the  Jerusalem 
artichoke,  the  dahlia,  and  the  com- 
mon white  water  lily  develop  tubers. 
The  scale  leaves  of  the  ordinary 
rootstock  are  in  tubers  reduced  to  ridges,  and,  the  buds  them- 
selves to  mere  points.  The  scales  and  buds  together  form 
the  eyes  of  tubers.  Tubers  serve  the  same  purposes  for  the 
plant  as  do  other  fleshy  underground  stems :  surplus  food 
accumulates  in  them,  and  by  them  the  plant  is  multiplied. 
The  potato  tuber  has  become  one  of  the  most  important 
sources  of  food  for  man. 

Commercial  products  derived  from  stems.  We  are  all 
familiar  with  the  important  products  derived  from  the  trunks 
of  trees.  Lumbering  is  one  of  the  most  important  industries 
of  the  United  States.  Closely  associated  with  it  are  the 
furniture  industry,  which  uses  the  hardwoods,  —  walnut, 
oak,  maple,  sycamore,  and  birch,  —  and  the  wood-pulp  in- 
dustry, which  utilizes  soft  woods  —  such  as  spruce  and 
poplar  —  in  the  making  of  paper.  The  Southern  pines  fur- 
nish rosin  and  turpentine ;  the  bark  of  oaks  and  hemlocks  sup- 
plies tannic  acid  for  the  manufacture  of  leather ;  and  Span- 


162 


Science  of  Plant  Life 


U.  S.  Forest  Service 


FIG.  02.  Southern  long-leaf  pines  tapped  for  resin. 


Ecological  Groups  of  Stems 


ish  oak  bark  provides  the 
cork  of  commerce. 

The  stem  of  the  rattan 
palm,  which  is  one  of  the 
very  longest  of  tropical 
climbers,  furnishes  the  cane 
for  chairs  and  materials  for 
basketry  and  wicker  furni- 
ture. In  this  country 
baskets  and  furniture  are 
made  from  shoots  of  the 
osier  willows. 

Stem  vegetables  of  im- 
portance as  food  for  human 
beings  include  the  potato, 
Jerusalem  artichoke,  as- 
paragus, dasheen,  and 
kohl-rabi. 

Sorghum  and  sugar  cane 
furnish  a  considerable  part 
of  the  sugar  and  sirup  of 


Bureau  of  Agriculture,  P.  7. 
FIG.  93.  Tapping  para  rubber  trees,  in  the 
Malay  States,  to  obtain  the  milky  juice  from 
which  crude  rubber  is  made. 


The  maple  is  the 


commerce, 
source  of  a  delightfully  flavored  sugar. 

The  crude  rubber  used  in  the  manufacture  of  shoes,  gar- 
ments, and  tires  is  furnished  by  the  stem  of  the  desert  plant, 
guayule,  and  the  sap  of  various  tropical  rubber  trees.  Many 
substances  used  in  medicine  are  derived  from  stems  directly 
or  by  distillation.  Wintergreen  oil  may  be  obtained  from  the 
twigs  of  the  sweet  birch,  and  camphor  from  the  stem  and 
branches  of  the  camphor  tree. 

The  bast  fibers  of  flax,  jute,  and  hemp  furnish  material  for 
the  manufacture  of  linen,  cordage,  coarse  fabrics,  and  rugs. 


164 


Science  of  Plant  Life 


Bureau  of  A  gricullure,  P.  I. 
FIG.  94.     Marketing  bamboo,  Philippine  Islands. 

Cornstalks  not  only  furnish  food  (ensilage)  for  cattle,  but  from 
them  is  made  the  packing  used  inside  the  armored  walls  of 
war  vessels. 


PROBLEMS 

1.  What  tree  is  most  valuable  to  the  natives  of  the  tropics?     Why? 

2.  Where  are  the  principal  forest  reserves  of  the  United  States  located?     May 
trees  be  lumbered  on  forest  reserves? 

3.  What  kinds  of  work  are  carried  on  by  the  United  States  Forest  Service  ? 

4.  Will  twining  stems  twine  about  a  horizontal  wire  ? 

5.  For  your  vicinity,  make  a  list  of  the  plants  that  climb  (i)  by  twining ;  (2)  by 
means  of  tendrils ;  (3)  by  means  of  roots. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Fourteen 

1.  Demonstration  of  imbibition  with  gelatin,  wood,  and  a  dry 
leaf. 

2.  Demonstration  of  osmosis  with  thistle  tube  and  animal  mem- 
brane ;  or  with  diffusion  shell  or  porous  cup ;  or  with  carrot  root 
and  glass  tube. 

3.  Study  the  root  system  of  a  seedling  or  'weed.     Note  primary, 
secondary,  and  adventitious  roots.     Make  a  drawing  of  the  root 
system. 

4.  In  the  field,  study  the  roots  of  some  plants  growing  in  dry 
sand ;  in  clay ;  and  in  swampy  low  ground. 

5.  Study  distribution  and  development  of  root  hairs  in  seedlings. 
Wheat  and  oat  seedlings  are  very  satisfactory  for  this  purpose. 

6.  With  a  microscope  examine  a  young  root.     Try  to  make  out 
the  root  cap ;  the  epidermis ;  the  cortex ;  the  vascular  axis. 

7.  Longitudinal  sections  of  a  root  tip  will  show  stages  in  cell 
division  and  cell  enlargement.     Note  changes  in  nucleus,  cytoplasm, 
and  vacuole. 


165 


CHAPTER  FOURTEEN 

THE  STRUCTURES  AND  PROCESSES  OF  ROOTS 

IN  preceding  chapters  we  have  learned  that  leaves  manu- 
facture food  in  the  presence  of  light ;  that  their  exposure  to 

air  facilitates  the  entrance  and 
exit  of  carbon  dioxid  and  oxy- 

Sen;  and  that  by  beinS  raised 
and  displayed  on  erect  stems 
their  efficiency  is  increased. 

The  support  of  leaves  by 
stems  makes  necessary  the  de- 
velopment of  mechanical  tissue 
in  the  stems.  The  display  of 
leaves  high  above  the  water 
supply  of  the  soil  requires  a 
conductive  system  capable  of 
FIG.  95.  Dandelion  plant,  showing  the  raising  water  and  mineral  salts 

primary  tap  root  and  its  branches,  the  from  tne  roots  ancj  of  carrying 
secondary  roots. 

food    away    from    the    leaves. 

Consequently  plants  that  expose  great  numbers  of  leaves  to 
the  light  must  develop  large  and  strong  stems.  The  stems, 
in  turn,  must  be  firmly  anchored  in  the  soil,  and  they  must 
also  be  supplied  with  the  water  and  mineral  substances  that 
press  up  through  them  to  the  leaves.  Anchorage  and  ab- 
sorption are  the  particular  functions  of  roots,  though  they 
carry  on  other  processes  also,  such  as  conduction  of  water, 
transfer  of  food  materials,  accumulation  of  food,  respiration, 
and  growth. 

Classification  of  roots.  The  root  of  a  well-developed  bean 
seedling  will  show  the  essential  features  of  roots.  There  is 
the  primary  root,  extending  downwards  from  the  base  of  the 

1 06 


The  Structures  and  Processes  of  Roots 


stem.  On  its  sides  are  numerous  secondary  roots  which 
extend  at  right  angles,  or  grow  obliquely  downwards.  Un- 
like stems,  roots  possess  no 
definite  nodes  from  which 
branches  arise.  A  second- 
ary root  may  originate  at 
any  point  on  the  primary 
root. 

In  many  seedlings  there 
are  also  roots  that  develop 
from  the  first  node  of  the 
stem.  All  roots  arising 
from  stems  and  leaves  are 
called  adventitious  roots. 
The  "prop  roots"  that 
develop  from  the  lower 
nodes  of  corn  stems  and 
the  roots  that  grow  from 
"  cuttings "  are  familiar 
examples.  Adventitious 
roots  develop  also  from  the 
stems  of  many  plants  like 
the  poison  ivy  and  trumpet 

Creeper  and  act  as  holdfasts  pIG.  96.  Stages  in  the  development  of  a  corn 
in  Supporting  these  Climbers  seedling.  P  is  the  primary  root,  S  a  secondary 

root,  and  A  an  adventitious  root  from  the  first 

on  trees  and  walls.    Adven-  node  of  the  stem, 
titious  roots  may  arise  also 

at  any  point  on  a  primary  or  secondary  root,  following  in- 
juries. For  example,  when  we  plant  pieces  of  horseradish 
or  dandelion  roots,  adventitious  roots  develop.  During  the 
dry  season  in  deserts  the  younger  parts  of  the  root  systems 


i68 


Science  of  Plant  Life 


of  some  plants  like  the  cactuses  are  killed,  and  when  the  next 
wet  season  comes,  many  adventitious  roots  develop  from  the 
parts  of  the  root  system  still  alive. 
In  desert  plants  these  new  adven- 
titious roots  do  most  of  the  absorb- 
ing work  during  the  moist  season. 

Root  hairs.     The  young  roots  of 
land  plants  generally  bear  root  hairs. 
These  are  delicate  elongations  of  the 
epidermal  cells  of  the  root.     They 
are  especially  concerned 
with   the  absorption  of   Iff 
water  and  mineral  salts, 
and    their   presence   in- 
creases    the     absorbing 
surface  of  the  root  from 
two  to  fifty  times.    Since  T 

*  FIG.  97.     Stages  m  the  growth  of  an  onion  seed- 

the     rate     Of     absorption  ling,  showing  the  lifting  and  shedding  of  the  seed 

depends     in     part     Upon  coats  and  the  development  of  the  primary  and 

.  secondary  roots. 

the  surface  area  in  con- 
tact with  the  soil  water,  the  advantage  in  root  hairs  is  evident. 
Root  hairs  are  usually  short-lived  structures,  their  duration 
being  best  measured  in  days.  They  begin  to  develop  at  a 
short  distance  from  the  tip  of  the  root.  Farther  back  they 
have  attained  full  length,  and  beyond  this  they  are  in  a  dy- 
ing or  dead  condition.  Thus  from  day  to  day  the  zone  of 
root  hairs  moves  forward  with  the  growth  in  length  of  the 
root.  This  brings  the  root  hairs  continually  in  contact  with 
new  supplies  of  water  and  food  materials  in  the  soil.  As  a 
plant  enlarges,  its  root  system  becomes  more  complex  through 
repeated  branching  and  the  elongation  of  the  branches.  Most 


The  Structures  and  Processes  of  Roots         169 


of  the  absorption  occurs  in  the  root-hair  zone,  and  this  is 
continually  moved  farther  and  farther  from  the  base  of  the 
stem.  In  large  trees  this  zone  may 
be  many  feet  from  the  base  of  the 
trunk. 

Advantages  in  spreading  roots. 
The  spreading  of  the  root  branches 
in  all  directions  not  only  enables 
the  roots  to  come  in  contact  with 
more  water  and  mineral  salts,  but 
as  the  woody  tissue  develops,  it 
helps  to  anchor  the  plant  more 
firmly.  It  is  an  interesting  fact 

that  when  stems  are  subjected  to 

bending  by  winds  or  other  agencies, 

the  mechanical  tissue  of  the  root 

develops  to  a  greater  extent  than 

ordinarily. 

The  principal   tissues  of   roots. 

In  the  root,  the  water-conducting 

tissue    and    the    wood    form    the 

central  axis.     Surrounding  this  is 

a  layer  of  food-conducting  tissue. 

In  perennial  roots  there  is  a  cam- 
bium   layer   between    the   central  FIG.  98.   Enlarged  view  of  the  end 

axis  and  the  food-conducting  tissue.  of  a  root'  showin& root  caP>  growing 

^    ,    .  j      , ,       f       ,  ,  , .  region,  and  root  hairs. 

Outside  the  food-conducting  tissue 

is  the  cortex,  extending  to  the  epidermis.  Perennial  roots 
like  those  of  trees  soon  lose  their  epidermis ;  later  the  cortex 
also  disappears.  The  continued  thickening  of  the  wood  and 
of  the  water-conducting  and  food-conducting  tissues  results 


Growing 
point 


.Region,  of 
elongation 


Root  cap 


170 


Science  of  Plant  Life 


in  the  death  of  the  outermost  layers  of  the  root  and  the 
formation  of  a  bark  very  similar  to  that  of  tree  trunks. 

Absorption.  The 
passage  of  water  and 
other  substances  into  a 
body  is  called  absorp- 
tion. Of  all  the  pro- 
cesses that  take  place 
in  plants,  absorption  is 
the  one  most  commonly 
associated  with  roots. 
In  addition  to  water, 
roots  absorb  the  min- 
eral substances  found 
tissue'' 


" ;  W-tft/T  'Epidermis 
Cortex. 

Rod-con- 
ducting 


Water-con- 
ducting 
vessel 


Growing 
point 


in  plants,  and  they 
absorb  a  part,  or  all, 
of  the  oxygen  needed 
for  respiration  by  their 
own  cells.  Water  and 
other  substances  not 
only  enter  the  external 
cells  of  a  plant  by  ab- 
sorption, but  they  pass 
from  cell  to  cell  within 
the  plant  by  the  same 
process.  A  dead  root 
in  the  soil  may  take 
up  water  and  become 
saturated;  but  only  a 
living  root  can  absorb  water  rapidly  enough  to  furnish  an 
adequate  supply  to  the  living  parts  above  the  soil.  The 


Root  cap 


FIG.  99.     Diagram  of  a  root  tip,  showing  the 
tissues  and  their  arrangement. 


The  Structures  and  Processes  of  Roots         171 

water  supply  of  plants,  therefore,  depends  upon  the  presence 
of  living  cells  in  the  roots. 

Three  physical  processes  are  involved  in  absorption  in 
plants.  These  processes  are  diffusion,  imbibition,  and  osmosis. 
Their  action  may  be  demonstrated  to  a  large  extent  by  the 
use  of  physical  apparatus. 

Diffusion.  If  a  small  dish  of  ether  is  exposed  in  a  room,  in 
a  few  minutes  the  odor  of  the  ether  may  be  noticed  in  all 
parts  of  the  room.  Even  if  there  were  no  air  currents,  the 
ether  would  evaporate ;  that  is,  particles  of  ether  would  rise 
from  the  surface  of  the  liquid,  pass  out  of  the  dish,  and  move 
through  the  room  in  every  direction.  This  is  an  example  of 
the  diffusion  of  a  vapor.  The  vapor  is  concentrated  in  the 
dish  and  the  particles  move  outward  into  the  room  where 
there  is  none ;  that  is,  the  particles  move  from  the  place 
where  the  concentration  is  greatest  to  where  it  is  less.  After 
the  ether  has  evaporated,  the  vapor  tends  to  become  evenly 
distributed  throughout  the  room. 

Similarly,  if  a  few  crystals  of  copper  sulfate  are  placed  in 
the  bottom  of  a  vessel  of  water,  particles  of  the  copper  sulfate 
diffuse  through  the  water.  The  crystals  are  blue  in  color, 
and  as  diffusion  proceeds,  the  water  in  the  vessel  gradually 
becomes  blue.  The  direction  of  the  movement  is  again  from 
the  place  where  the  diffusing  substance  is  most  concentrated 
to  where  it  is  less  concentrated.  The  particles  pass  from  the 
place  where  they  are  most  abundant  to  where  there  are  fewer 
of  them,  and  this  process  is  continued  until  they  are  evenly 
distributed  throughout  the  water. 

Diffusion  of  a  gas  or  vapor  is  very  rapid.  Diffusion  of  a 
dissolved  substance  is  slow,  but  the  distances  that  substances 
must  travel  in  plant  cells  are  very  small.  Oxygen  and  carbon 


172  Science  of  Plant  Life 

dioxid,  when   once   dissolved   in   the   water  of   cells,  move 
about  partly  by  diffusion. 

Imbibition.  The  process  of  imbibition  may  be  illustrated  by 
placing  a  sheet  of  gelatin  in  water.  Dry  gelatin  is  a  hard, 
brittle,  partly  transparent  solid.  After  it  has  been  in  water 
for  a  few  minutes,  it  will  be  found  to  have  increased  in  weight 
and  in  length,  breadth,  and  thickness.  The  gelatin,  instead 
of  being  brittle,  is  now  soft  and  pliable ;  it  is  also  more  trans- 
parent than  it  was. 

The  increase  in  size  and  weight  is  explained  by  the  fact  that 
particles  of  water  have  forced  their  way  between  the  particles 
of  the  gelatin,  spreading  them  apart.  Since  the  gelatin  par- 
ticles have  been  forced  farther  apart,  the  gelatin  is  more  pli- 
able and  the  particles  cling  to  one  another  less  firmly.  Hence 
when  a  piece  of  dry  wood  is  put  into  water,  it  imbibes  water 
and  swells.  The  cell  walls  of  a  root,  like  wood,  are  largely 
composed  of  cellulose  and  they  take  up  water  in  the  same  way. 
When  dry  seeds  are  placed  in  water,  they  imbibe  water  and 
increase  in  size.  Indeed,  most  organic  substances  have 
the  property  of  imbibing  water  and  swelling.  Imbibition  is 
a  form  of  diffusion  that  results  in  swelling.  Compare  the 
size  of  a  sponge  when  dry  with  its  size  after  it  has  been  soaked 
in  water  and  squeezed  as  dry  as  possible. 

When  a  piece  of  wood  becomes  saturated,  it  stops  taking 
up  water.  If,  however,  the  water  were  being  removed  from 
the  inside,  more  would  continue  to  pass  into  the  wood.  This 
is  exactly  what  happens  in  the  root  of  a  living  plant.  The 
external  cells  of  the  root  are  in  contact  with  the  water  of  the 
soil.  Inside  the  root  the  water  is  being  used  and  removed  by 
being  drawn  up  through  the  stem  to  the  leaves.  More  water 
then  passes  into  the  cell  walls  and  protoplasm  to  take  the  place 


The  Structures  and  Processes  of  Roots          173 

of  that  which  is  drawn  away,  and  this  keeps  the  amount  of 
water  in  the  cells  of  the  root  nearly  constant. 

Osmosis.  The  third  physical  process  that  aids  in  the 
absorption  of  water  is  osmosis.  If  an  animal  membrane,  as 
a  piece  of  bladder,  is  tied  over  the  broad  end  of  a  thistle  tube 
and  the  bulb  of  the  tube  is  immersed  in  water,  the  water  will 
gradually  pass  through  the  membrane.  The  membrane  is 
permeable  to  water ;  that  is,  it  allows  water  to  pass  through 
its  minute  pores.  The  water  continues  to  move  through  until 
its  level  is  the  same  inside  and  outside  (Fig.  100). 

When  the  water  level  is  the  same  inside  and  outside  the 
tube,  one  might  think  that  the  water  particles  were  at  rest. 
This  is  not  the  case.  Water  particles  are  still  passing  both 
into  the  thistle  tube  and  out  -of  it  through  the  membrane. 
The  rate  is  the  same  in  both  directions,  however,  and  so  the 
water  level  within  the  tube  remains  unchanged. 

If  we  put  a  little  sugar  into  the  thistle  tube,  something  dif- 
ferent happens,  as  is  shown  by  the  fact  that  the  liquid  in  the 
tube  begins  to  rise.  Evidently,  more  water  is  passing  through 
the  membrane  into  the  tube  than  is  passing  out,  and  this 
change  has  been  brought  about  by  the  presence  of  the  sugar. 
Perhaps  we  can  get  a  mental  picture  of  what  causes  this  dif- 
ference from  the  diagram  in  Figure  101.  The  membrane  (C) 
allows  water  molecules  to  pass  through  it  freely,  but  it  per- 
mits scarcely  any  of  the  sugar  molecules  to  pass.  The  outer 
side  of  the  membrane  is  completely  covered  with  water  mole- 
cules (B),  tending  to  diffuse  through  the  membrane.  The 
inner  side  (A)  is  only  partly  covered  with  water  molecules, 
since  part  of  the  area  is  occupied  by  sugar  molecules.  Con- 
sequently there  are  fewer  water  particles  on  the  side  A  tend- 
ing to  diffuse  outward  than  there  are  on  the  side  B  tending  to 


174 


Science  of  Plant  Life 


diffuse  inward.     We  may  say  that  the  water  is  more  con- 
centrated outside  the  tube  than  inside,  so  water  passes  from 


~  -  ~WATER~    - 


~  -=1  WATER  **• 


-    WATER+SUGARJ       — 


/    •  li/  A  T-rr  r*  D 


FIG.  100.  Diagrams  to  illustrate 
the  passage  of  water  through  a 
membrane :  A,  molecule  of  inside 
water;  B,  molecule  of  outside 
water;  C,  membrane. 


FIG.  101.  Diagram  to  illustrate 
osmosis:  A,  sugar  molecule; 
B,  water  molecule;  C,  selectively 
permeable  membrane. 


the  place  of  greater  concentration  to  the  place  of  less  con- 
centration. Moreover,  sugar  is  a  highly  soluble  substance ; 
that  is,  it  has  a  great  affinity  for  water,  and  the  sugar  particles 
tend  to  hold  the  water  particles  in  contact  with  them  inside 
the  thistle  tube.  The  sugar,  like  the  water,  tends  to  pass  from 
the  place  of  greatest  concentration  but  is  restrained  by  the 
membrane  from  moving  outward. 

If  we  close  the  upper  end  of  the  thistle  tube,  the  water  will 
continue  to  rise  and  compress  the  inclosed  air.  The  pres- 
sure developed  under  these  conditions  is  called  osmotic  pres- 
sure. If  a  large  amount  of  sugar  is  put  inside  the  tube,  the 
water  will  rise  rapidly  and  exert  great  pressure.  If  only  a 


The  Structures  and  Processes  of  Roots          175 

small  amount  of  sugar  is  present  inside  the  tube,  the  water 
will  rise  slowly  and  exert  but  little  pressure. 

When  a  membrane  permits  water  or  other  substances  to 
pass  through,  it  is  said  to  be  permeable  to  that  substance. 
For  example,  animal  membranes  are  permeable  to  water  and 
to  various  dyes.  A  membrane  that  allows  one  substance  to 
pass  through  it,  but  retards  the  passage  of  another  substance, 
is  said  to  be  selectively  permeable.  The  membrane  on  the 
thistle  tube  is  selectively  permeable,  because  it  allows  the 
passage  of  water  but  restrains  the  sugar  that  is  dissolved  in 
the  water.  The  membranes  in  root  cells  are  permeable  to 
water,  but  they  do  not  allow  sugar  and  many  other  sub- 
stances found  inside  the  cells  to  pass  out. 

The  conditions  for  osmosis  as  it  occurs  in  plants,  then,  in- 
clude a  selectively  permeable  membrane  between  two  bodies  of 
water,  one  of  which  contains  a  dissolved  substance  that  does  not 
pass  through  the  membrane  readily. 

Osmosis  in  roots.  The  cellulose  walls  of  plant  cells  are 
permeable  to  water  and  to  most  of  the  substances  that  dis- 
solve in  water ;  but  the  layer  of  cytoplasm  inside  the  cell  wall 
forms  a  selectively  permeable  membrane  about  the  cell  con- 
tents. The  cell  sap  in  the  vacuole  may  contain  sugar  and  other 
dissolved  substances,  just  as  the  water  in  the  thistle  tube  con- 
tained sugar.  The  outer  cells  of  the  root  are  in  contact  with 
the  soil  water.  Hence  the  water  passes  into  these  cells  in 
the  same  way  as  into  the  thistle  tube.  In  a  similar  manner 
water  may  move  from  one  cell  to  another  and  replace  the 
water  that  is  being  carried  to  the  stem  and  leaves  through  the 
conductive  tissue.  The  path  of  the  water  from  the  epidermal 
cells  is  through  the  cells  of  the  cortex  to  the  water-conducting 
vessels  in  the  interior  of  the  root. 


176 


Science  of  Plant  Life 


Absorption  and  rise  of  sap  in  plants.     Attention  was  called 
to  the  fact  that  transpiration  exerts  a  pull  on  the  water  in 

the  conducting  tissue  of  the 
leaves  (page  134).  This  pull  is 
transmitted  to  the  water-con- 
ducting tissues  of  the  stem  and 
root.  So  a  fourth  factor  enters 
into  the  absorption  of  water  by 
the  roots  :  the  pull  on  the  water 
in  the  cells  of  the  root  is  in- 
directly due  to  transpiration 
from  the  leaves.  Large  trees 
have  been  kept  alive  for  days 
by  placing  the  cut-off  trunks  in 
water.  This  shows  that  suffi- 
cient water  to  maintain  the 
water  balance  of  the  plant  for 
at  least  several  days  may  be 
lifted  in  a  plant  by  the  pull  of 
transpiration  without  the  aid  of 
roots.  It  is  of  practical  interest 

FIG.  102.     Expenment  to  illustrate  the 

water  balance  in  a  plant.     The  entire    to  knOW  that  CUt  flowers  Will  last 


apparatus  is  filled  with  water,  and  A 
and   C  are  immersed  in  water.     The 


jf 


ends  Qf 


. 

water  is  absorbed  by  osmosis  into  the  stems  are  bent  over  into  a  vessel 

porous  cup  A,  and  evaporated  from  the  an(J  cut  un(ier  the  Water.  If  CUt 
cup  B.  The  rate  of  evaporation  is 

faster  than  the  rate  of  absorption,  as  is  in  the  air,  air  bubbles  get  into 
shown  by  the  fall  of  the  mercury  in  the  ^he  Water-Conducting  tubes  and 
outer  end  of  the  tube  C. 

prevent   the  subsequent  move- 

ment of  water  into  them.  Air  bubbles  already  in  stems  that 
have  been  cut  in  the  air  may  sometimes  be  removed  by  cutting 
off  an  inch  or  two  of  the  lower  ends  of  the  stems  under  water, 


The  Structures  and  Processes  of  Roots 


177 


Root  pressure.     If  a  number  of  well- watered  plants  are  cut 
off  just  above  the  soil,  some  of  them  will  exude  water  for  a 
day  or  two.     Experiments  have 
shown  that  the  sap  may  in  some 
cases  be  forced  out  with  pres- 
sure sufficient  to  raise  water  30 
or   40   feet.      This   pressure   is 
called  root  pressure.    When  such 
pressures  exist  in  plants,   they 
probably  aid  in  the   lifting  of 
water  in  stems.    Under  these  cir- 
cumstances   transpiration   pulls 
on  the  columns  of  water  in  the 
water-conducting    vessels    from 
the  top,  and  root  pressure  pushes 
on  them  from  below.     Extensive 
experiments  have  shown,  how- 
ever, that  root  pressure  is  inter-  -J>------ 

mittent.     It  may  exist  at  one  FlG"  I0f   A  plant  with  its  stem  cut  in 

two  and  connected  again  with  a  tube 
time    and    not    at    another,     and   similar   to   that  shown  in  Figure  102. 

when  transpiration  is  most  active  In  this  case  the  roots  are  absorbins 

water  more  rapidly  than  the  leaves  are 

and  the  largest  Volumes  Of  Water  transpiring  it,  since  the  mercury  at  D  is 
are  being  raised  in  a  plant,  root  Pushed  away  from  the  plant.     By  set- 
ting the  plant  in  bright  sunshine,  the 

pressure     is    wanting    entirely.  transpiration  may  be  increased.    The 

Because   of    all    these    facts,  it  is  mercury   is    then    almost    immediately 

generally     believed     that     root  d™- toward  the  plant. 

pressure  is  not  a  necessary  factor  in  the  raising  of  water  in 

stems. 

Imbibition  and  osmosis  lead  to  the  development  of  root 
pressure,  and  they  are  partly  responsible  for  the  flow  of 
maple  sap  (page  135).  Grapevines  pruned  in  the  spring  exude 


178  Science  of  Plant  Life 

water  for  days  afterward  as  a  result  of  root  pressure.  On  a 
small  scale  the  same  thing  may  some  times  be  seen  when  well- 
watered  begonias  and  fuchsias  are  cut  off  near  the  soil. 

Food  conduction.  The  transfer  of  food  takes  place  in  the 
food-conducting  tissue  of  roots  in  the  same  way  as  in  stems 
and  leaves.  Substances  that  are  to  be  transferred  must  be 
in  a  soluble  form,  and  they  are  usually  in  a  comparatively 
simple  form.  Starch,  for  example,  is  transferred  as  glucose, 
and  protein  and  fats  are  broken  down  into  simpler  compounds 
before  they  are  moved  from  one  part  of  the  plant  to  another. 
Diffusion  is  the  principal  process  that  brings  about  food 
conduction. 

The  movement  of  a  substance  into  or  out  of  a  cell  depends 
upon  the  permeability  of  the  cell  protoplasm  to  that  par- 
ticular substance ;  if  the  cytoplasm  will  not  permit  the  sub- 
stance to  pass  through,  it  cannot  enter  or  leave  a  cell.  The 
direction  of  the  movement  of  foods  may  change  from  time  to 
time,  as  is  shown  by  the  fact  that  sugar  and  soluble  proteins 
may  move  down  into  the  root  during  one  season  and  up  out 
of  the  root  at  another  season.  For  example,  in  the  turnip  or  beet 
the  excess  food  made  by  the  leaves  during  the  first  summer 
passes  downward  into  the  roots ;  the  next  year,  food  passes 
upward  from  the  roots  to  the  developing  stems  and  leaves. 
This  may  be  due  to  changes  in  the  permeability  of  the  cells  or 
to  changes  in  the  foods  stored  in  the  cells. 

These  changes  in  the  behavior  of  organs,  tissues,  and  cells 
are  clear  evidences  of  life.  In  physical  apparatus  the  behavior 
is  fixed  and  a  process  soon  comes  to  a  standstill.  In  living 
things  changes  are  continually  taking  place  in  the  living  mat- 
ter itself,  and  these  bring  about  continual  changes  in  the 
processes  that  are  going  on. 


The  Structures  and  Processes  of  Roots          179 

Accumulation  of  food  in  roots.  Food  accumulates  in  the 
roots  of  many  plants,  notably  in  those  of  biennials  like  the 
beet,  carrot,  turnip,  and  salsify.  The  sweet  potato  and  the 
dahlia  are  examples  of  perennials  with  large  storage  roots. 
The  most  common  forms  in  which  carbohydrates  accumulate 
in  roots  are  starch  and  sugar.  Starch  as  a  storage  material 
has  the  advantages  of  being  insoluble  and  more  concentrated 
than  sugar.  When  growth  begins  anew,  starch  is  readily 
converted  (digested)  into  sugar  (page  74). 

Respiration  in  roots.  Respiration  must  go  on  in  the  living 
cells  of  the  roots  just  as  in  the  other  living  parts  of  the  plant. 
This  process  requires  a  constant  supply  of  oxygen.  In  ob- 
taining oxygen  as  well  as  in  obtaining  water,  the  division  of 
the  roots  into  numerous  fine  branches  is  an  advantage,  be- 
cause it  exposes  a  large  surface  to  the  soil  air  and  the  soil 
water.  Some  plants  are  easily  injured  by  the  lack  of  oxygen 
in  the  soil ;  if  water  stands  on  the  soil  and  excludes  the  air, 
the  roots  gradually  suffocate.  Suffocation  of  a  part  of  the 
roots  interferes  with  other  root  processes  besides  respiration, 
and  the  whole  plant  suffers.  For  example,  you  may  have 
seen  yellow,  sickly  corn  in  low  fields  where  water  has  stood 
for  some  time.  Such  plants  may  recover  if  the  soil  is  drained. 
Water  plants  and  swamp  plants  can  grow  in  poorly  aerated 
soils  because  the  roots  are  able  to  secure  oxygen  through  the 
internal  air  spaces  of  the  plants. 

Under  normal  conditions  the  energy  liberated  by  respira- 
tion in  roots  is  largely  used  in  growth  and  in  overcoming  the 
resistance  of  the  soil.  During  the  life  of  a  plant  the  roots, 
like  the  stem,  continue  to  develop  new  branches. 

The  growth  of  roots.  In  growing  through  the  soil,  the  tip 
of  a  root  is  continually  pushed  against  and  between  sharp- 


180  Science  of  Plant  Life 

angled  soil  particles.  If  a  root  tip  or  a  longitudinal  section 
of  one  is  examined  under  a  microscope,  it  may  readily  be  seen 
that  the  growing  point  is  not  at  the  very  end,  as  in  stems, 
but  is  covered  by  a  root  cap.  The  growing  region  of  the  root 
extends  a  few  centimeters  back  from  the  growing  point  (Fig.  98) . 
Therefore,  as  the  root  elongates,  the  root  cap  is  pushed  for- 
ward and  is  abraded  by  the  soil  particles.  This  injures  the 
outer  cells  of  the  cap,  but  as  they  are  being  renewed  from 
within,  the  cap  is  always  present  as  a  protection  for  the  grow- 
ing point. 

At  the  growing  point  the  cells  are  small,  similar  in  form,  and 
filled  with  protoplasm.  They  are  constantly  dividing  and 
forming  new  cells.  As  the  cells  grow  older  and  enlarge,  they 
assume  the  mature  cell  form  of  the  particular  tissue  to  which 
they  belong.  The  protoplasm  of  mature  cells  is  merely  a 
lining  inside  the  cell  wall.  Most  of  the  cell  space  is  occupied 
by  a  water  solution  of  sugar,  soluble  proteins,  salts,  and  acids. 
Sometimes  starch  grains  are  present  (Fig.  99,  page  170). 

The  three  stages  of  growth  are  characterized  by  cell  division, 
cell  enlargement,  and  the  fixation  or  thickening  of  the  cell 
walls.  They  are  very  similar  in  all  plant  organs.  But  no- 
where can  they  be  seen  so  readily  as  in  the  longitudinal  section 
of  a  young  root. 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Fifteen 

1.  Grow  seedlings  on  blotting  paper  or  cotton,  between  two 
glass  plates.     Keep  the  plates  in  one  position  until  the  roots  are 
2  or  3  inches  long.     Then  set  the  plates  at  right  angles  to  the 
previous  position.     Note  the  change  in  the  direction  of  growth  of 
primary  and  of  secondary  roots. 

2.  Replace'  the  bottom  of  a  cigar  box  with  mosquito  netting, 
and  place  on  the  netting  mustard  or  radish  seeds  that  have  pre- 
viously been  soaked  in  water.     Then  fill  the  box  with  wet  moss  or 
cotton.     Suspend  the  box  so  that  the  bottom  rests  at  an  angle  of 
45  degrees.     Note  the  direction  of  growth  of  the  roots  as  they 
develop.     Do  they  respond  more  to  gravity  or  to  moisture  ? 

3.  Study   roots   of   floating   or   submerged  plants.     Note   air 
chambers. 

4.  Study  aerial  roots  of  climbers  like  Boston  ivy  and  trumpet 
creeper.     Study  the  prop  roots  of  mature  corn. 

5.  Sketch  and  study  roots  of  carrot,  parsnip,  or  radish.     Test 
for  starch. 

6.  Sketch  and  study  root  nodules  and  roots  of  either  clover  or 
beans. 

7.  Study  the  arrangement  of  roots  on  a  bulb  or  corm.     Onion, 
hyacinth,  gladiolus,  or  crocus  may  be  used. 


181 


CHAPTER   FIFTEEN 


ROOTS   AND   THEIR  ENVIRONMENT 

ROOTS  commonly  develop  in  soil.     Floating  plants,  however, 
develop  roots  in  water ;  and  many  plants  in  the  moist  tropics 

develop  roots  in  the  air.  Some 
of  our  own  climbing  plants  have 
aerial  roots.  Corresponding 
with  the  differences  in  their  en- 
vironments, there  are  marked 
differences  in  the  structures 
and  processes  of  roots. 

The  response  of  roots  to 
gravity.  The  downward 
growth  of  a  primary  root  is 
a  response  to  gravity.  The 
primary  root  is  positively  geo- 
tropic;  it  turns  toward  the 
earth.  We  must  clearly  un- 
derstand, however,  that  gravity 
does  not  pull  the  root  into  the 
soil.  The  soil  particles  are 
heavier  than  the  roots,  and 
this  means  that  gravity  pulls 
P.  more  on  them  than  it  does  on 

FIG.  104.    Young  and  old  stems  of  Bos- 
ton ivy.     The  young  stems  are  held  by    the    TOOtS.      A    TOOt    will     even 
means  of    tendrils,   the  older  stems  by  downward   into   mercury, 

means  of  adventitious  aerial  roots.  . 

which  is  15  times  as  heavy  as 

the  root.  Gravity  merely  determines  the  direction  of 
growth.  The  penetration  of  the  soil  is  due  to  growth  pres- 
sure ;  the  cells  of  the  root  multiply  and  enlarge,  forcing  the 
tip  downward.  Secondary  roots  also  respond  to  the  pull  of 

182 


Roots  and  Their  Environment  183 

gravity,  but  they  tend  to  grow  at  right  angles  to  its  direc- 
tion rather  than  directly  toward  it,  as  do  primary  roots. 
Secondary  roots  are  transversely  geo tropic. 

The  response  of  roots  to  light.  Roots  tend  to  grow  away 
from  strong  light.  They  are  negatively  photo  tropic.  This 
tendency  may  be  observed  in  the  aerial  roots  of  ivies  and 
other  climbers.  No  matter  from  what  surface  on  the  stem 
they  arise,  they  curve  around  away  from  the  light.  This  is 
advantageous  to  a  climbing  plant,  for  it  brings  the  root  to 
the  supporting  tree  or  wall. 

The  response  of  roots  to  water.  Particles  of  water,  in  dif- 
fusing from  wet  places  in  the  soil  to  places  that  are  drier,  also 
affect  the  direction  of  root  growth.  A  root  in  drier  soil  will 
turn  toward  the  direction  from  which  water  particles  strike 
it.  Continued  growth  will  then  bring  the  root  into  the 
moister  soil.  This  response  of  a  root  is  of  great  advantage 
to  the  plant.  It  is  sometimes  stated  that  roots  "  seek  "  the 
water.  In  reality,  roots  grow  toward  moist  places  only  when 
water  diffusing  from  the  moist  places  reaches  them  and  so 
controls  the  direction  of  growth.  They  do  not  seek  the  water, 
but  they  are  turned  to  the  moist  soil  by  the  water  itself. 

The  distribution  of  roots  in  the  soil.  Another  factor  that 
determines  the  distribution  of  roots  in  soil  is  the  oxygen  sup- 
ply. Various  plants  have  different  requirements,  but  all  roots 
doubtless  require  oxygen  for  growth.  Those  who  have  seen 
stumps  pulled  from  the  land  know  that  the  roots  go  deep  in 
upland  sandy  soils ;  that  they  do  not  go  so  deep  in  heavy  clay 
soils ;  and  that  they  are  just  beneath  the  surface  in  swamp 
and  bog  land.  The  principal  reason  why  one  finds  the  roots 
near  the  surface  in  swamps  is  that  these  roots  were  the  only 
ones  that  continued  to  live  and  grow.  The  roots  that  in  times 


184  Science  of  Plant  Life 

of  drought  penetrated  to  greater  depths  were  killed  off  — 
suffocated  —  when  the  water  stood  at  higher  levels.  The 
distribution  of  roots  in  the  soil,  therefore,  is  determined  prin- 
cipally by  the  combined  influences  of  gravity,  water,  and 
oxygen.  Water  and  gravity  control  the  direction  of  growth, 
and  the  oxygen  supply  determines  whether  or  not  growth  can 
take  place  or  the  roots  survive. 

In  the  plains  of  eastern  Kansas,  the  roots  of  plants  may 
penetrate  certain  soils  to  depths  of  from  15  to  20  feet.  The 
absorbing  parts  of  these  roots  actually  reach  the  water  table ; 
that  is,  they  reach  the  level  at  which  the  soil  is  saturated, 
or  the  level  to  which  water  would  rise  in  a  well. 

Two  or  more  species  of  plants  are  sometimes  found  associ- 
ated in  dry  regions,  and  locally  in  dry  habitats,  because  their 
roots  get  their  water  at  different  levels  and  hence  do  not 
compete  with  each  other.  For  example,  in  our  Southern 
deserts  the  giant  cactus  commonly  grows  with  the  creosote 
bush.  The  former  plant  obtains  its  water  from  the  super- 
ficial layers  of  the  soil,  while  the  latter  obtains  its  water  at 
deeper  levels.  The  roots  of  lawn  grass  are  very  superficial, 
and  lawn  grass  suffers  from  drought  much  sooner  than  do 
the  deeper-rooted  dandelion  and  English  plantain  that  occur 
with  it  as  weeds. 

In  dry  regions  where  plants  compete  with  one  another,  suc- 
cess comes  mostly  to  those  that  secure  a  sufficient  water 
supply.  In  moist  regions  success  in  competition  between 
plants  depends  chiefly  on  ability  to  reach  the  light  or  with- 
stand shade. 

The  pressure  of  growth.  The  pressure  exerted  by  roots  in 
penetrating  the  soil  may  be  very  great,  amounting  to  hun- 
dreds of  pounds  to  the  square  inch.  This  is  readily  appreci- 


Roots  and  Their  Environment  185 

ated  when  one  sees  cement  sidewalks  broken  and  large  rocks 
moved  by  the  growth  of  roots  under  them.     Growth  pres- 


FIG.  105.     Fern  leaves  pushing  upward  through  a  cement  sidewalk. 
(After  G.  E.  Stone.} 

sure  is  just  as  powerful  in  stems  and  other  growing  parts. 
Fleshy  roots  like  those  of  the  radish  and  turnip  sometimes 
force  themselves  partly  out  of  the  ground  by  the  thickening 
of  the  upper  portion. 

Root  contraction.  As  roots  mature,  they  may  contract 
in  length  and  so  draw  the  base  of  the  stem  a  slight  distance 
into  the  soil.  In  this  way  crevice  plants  on  cliffs  are  con- 
tinually held  firmly  in  place,  in  spite  of  the  wearing  away  of 
the  cliff  face  by  .erosion.  In  the  same  way  the  crowns  of 
clover  and  plantain  roots  that  have  been  lifted  up  by  frosts 
may  be  drawn  into  the  soil,  and  small  bulbs  and  tubers,  many 
of  which  are  formed  at  higher  levels  than  the  parent  bulbs, 
may  be  pulled  deeper  into  the  soil  by  root  contraction. 

Root  duration.  The  roots  of  various  plants  are  annual, 
biennial,  or  perennial.  Perennial  plants  may  have  either 
annual  or  perennial  roots,  just  as  they  may  have  either  an- 
nual or  perennial  aerial  stems.  Plants  with  bulbs,  tubers,  or 
corms  grow  a  new  set  of  roots  each  year.  Plants  with  root- 
stocks,  like  the  May  apple  and  Solomon's  seal,  generally 
have  roots  that  last  for  several  years.  Shrubs  and  trees  also 
have  perennial  roots.  We  must  be  sure  to  understand,  how- 


i86 


Science  of  Plant  Life 


U.  S.  Forest  Service 

FIG.  106.  Typical  section  of  a  mountain  slope  in  western  North  Carolina,  after 
removal  of  forest.  The  binding  effects  of  the  roots  have  been  removed,  and  the  erosion 
of  the  soil  is  so  rapid  that  it  is  difficult  for  seedlings  to  take  hold.  When  the  forest  was 
cut,  enough  young  trees  should  have  been  left  to  hold  the  soil  and  start  a  new  lumber  crop. 

ever,  that  even  in  perennial  roots  the  work  of  absorption  is 
for  the  most  part  done  by  the  new  roots  which  are  added  each 
year.  Most  biennials,  like  the  common  evening  primrose 
and  wild  carrot  (page  315),  have  fleshy  roots  in  which  food 
accumulates  during  the  first  year.  This  food  is  used  in  the 
rapid  development  of  the  plant  during  the  second  season. 

Ecological  types  of  roots.  Most  of  the  root  characteristics 
thus  far  described  are  those  of  the  roots  of  mesophytes.  In 
hydrophytes,  or  water  plants,  the  roots  are  notably  smaller 
and  less  branched  than  in  mesophytes.  They  absorb  water 
and  mineral  substances  from  the  soil  even  when  the  plants 


Roots  and  Their  Environment 


are  totally  submerged.  The  roots  of  hydrophytes,  like  the 
leaves  and  stems,  are  remarkable  for  the  presence  of  internal 
air  cavities  (page  156). 

When    the    roots    of 
land  plants  (mesophytes) 
extend  into  well-aerated 
water,  they  develop  in- 
numerable branches,  dif-      > 
fering    in    this    respect  -  • 
very  markedly  from  the 
roots    of     hydrophytes. 
On  account  of  this  fact, 
roots  of  trees,  especially 
those  of  willow  and  cot- 
ton wood,      that      enter       V 
drain    pipes    and    tiles 
often  develop  masses  of       i 
fine   branches   that  ob-    — ' 
struct   the   flow  of   the    ••- 
water    even    when    the 

entering  root  is  not  FlG  I07  Holdfast  roots  of  trumpet  creeper,  de- 
thicker  than  the  lead  in  veloped  from  the  nodes.  These  roots  are  perennial 

a  pencil.     The  banks  of  and  may  lengthen  and  branch  for  several  years* 
streams  are  often  protected  from  erosion  by  the  mat  of  roots 
developed  along  the  water's  edge.     This  is  why  willows  are 
planted  on  levees. 

In  moderately  dry  regions  the  roots  of  xerophytes  may 
penetrate  to  considerable  depths,  but  in  deserts  many  of  the 
largest  plants  have  only  small  root  systems,  spread  in  the 
upper  layers  of  the  soil  (page  184). 

Climbing  plants,   like  the  Virginia   creeper,   poison   ivy, 


i88 


Science  of  Plant  Life 


FIG.  i 08. 


Bureau  of  Science,  P.  I. 
Epiphytes  on  the  branches  of  trees  in  the  rainy  tropics. 


Roots  and  Their  Environment 


189 


Boston  ivy,  and  trumpet  creeper,  develop  holdfast  roots  which 
help  to  support  the  vines  on  trees,  walls,  and  rocks.  By 
forcing  their  way  into  minute  pores  and  crevices,  they  hold 
the  plant  firmly  in  place.  Usually  the  roots  die  at  the  end  of 
the  first  season,  but  in  the  trumpet  creeper  they  are  perennial. 
In  the  tropics  some  of  the  large  climbing  plants  have  holdfast 
roots  by  which  they  attach  themselves,  and  long,  cordlike 
roots  that  extend  downward  through  the  air  until  they  strike 
the  soil  and  become  absorbent  roots. 

Epiphytes.  A  plant  that  lives  perched  on  another  plant  is 
an  epiphyte  (Greek:  epi,  upon,  and  phyton,  plant).  Mosses 
and  lichens  are  the  most  common  epiphytes  in  temperate 
regions,  but  in  the  rainy 
tropics  and  along  our  own 
Southern  coast  many  flower- 
ing plants  live  attached  to  the 
branches  of  trees.  They  usu- 
ally have  leathery  leaves  and 
a  low  transpiration  rate. 
Many  have  water-storage 
tissue  in  fleshy  stems  or  in 
thickened  leaves.  Others  are 
called  tank  epiphytes,  because 
they  catch  water  in  the  axils  of 
the  leaves  or  in  pitcher-like 
leaves.  Epiphytes  cling  to 
the  supporting  tree  by  means 
of  roots  that  act  both  as  hold- 
fasts and  water-absorbing 

Organs.        They    do     not     take    ffIGh  J09'     Florida  epiphyte 

It  belongs  to  the  Uromelia  or  pineapple 

their   nourishment   from   the   family. 


190 


Science  of  Plant  Life 


plants  on  which  they  grow 
(page  249),  but  depend  for 
their  water  upon  the  evenly 
distributed  rainfall  and  for 
their  mineral  substances  upon 
dust  and  the  decay  of  the 
bark  on  which  they  live. 

Epiphytes  are  pronounced 
xerophytes,  for  there  is  prob- 
ably no  habitat  in  which  it 
is  more  difficult  to  maintain 
a  water  balance  than  the  one 
in  which  they  live.  It  is  not 
surprising,  therefore,  to  find 
that  among  the  epiphytic 
plants  of  the  West  Indies 
there  are  several  species  of 
cactus.  Among  epiphytes 

there  are  many  species  of  ferns,  and  many  species  belong 
to  two  families  of  flowering  plants,  the  Bromelias  and 
orchids.  The  Bromelias  are  related  to  the  pineapple  and 
have  leaves  of  the  same  type.  The  orchids  have  flowers 
remarkable  for  their  shapes  and  colors,  and  have  the  dis- 
tinction of  being  the  highest  priced  of  all  flowering  plants. 
The  long  moss  of  Florida,  a  flowering  plant,  is  perhaps  the 
best  known  of  American  epiphytes.  It  is  an  extreme  form 
and  is  devoid  of  roots.  The  roots  of  many  epiphytes  contain 
chlorophyll  and  assist  in  the  manufacture  of  food. 

Roots  and  transplanting.  Only  a  few  years  ago  it  was 
thought  impossible  to  transplant  large  trees  or  even  medium- 
sized  conifers.  Today  trees  of  large  size  are  dug  up,  trans- 


FIG.  no.     An  epiphytic  orchid. 


Roots  and  Their  Environment 


191 


U.  S.  Forest  Service 

FlG.  in.  Live-oak  tree  draped  with  Spanish  moss.  This  epiphyte,  which  is 
common  along  the  Gulf  Coast,  has  no  roots.  It  is  a  flowering  plant  belonging  to 
the  Bromelia  family. 

ported  many  miles,  and  replanted  successfully.  Even  whole 
hedgerows  several  feet  in  height  are  transplanted  without 
injury.  This  advance  in  the  art  of  tree  moving  is  a  fine  ex- 
ample of  the  application  of  a  knowledge  of  root  physiology 
to  practical  problems. 

We  have  learned  that  the  absorbing  part  of  the  roots  is 
mostly  in  the  root-hair  zone  near  the  root  tips.  Formerly 
when  a  tree  was  dug  up  for  transplanting,  all  the  roots  were 
cut  off  3  or  4  feet  from  the  base  of  the  stem.  This  operation 
destroyed  practically  all  the  absorbing  organs,  and  the  tree 
could  not  absorb  water  from  the  soil  until  a  new  set  of  roots 
had  developed.  Meanwhile  it  suffered  from  extreme  drought 
and  not  infrequently  died. 


192 


Science  of  Plant  Life 


FIG.  112.  By  using  great  care  to  pre- 
serve the  root  system  a  tree  may  be 
transplanted  even  when  in  leaf.  Some 
hours  before  the  tree  is  lifted  it  is  thor- 
oughly watered  in  order  that  the  leaves 
and  other  parts  may  have  in  them  as 
large  a  supply  of  water  as  possible. 


FIG.  113.  The  long  roots  are  carefully 
laid  bare,  leaving  a  ball  of  earth  close  to 
the  trunk  of  the  tree .  B  ef  ore  this  is  done 
the  hole  where  the  tree  is  to  be  planted 
should  be  made  ready.  It  should  be  so 
dug  that  the  tree  will  not  be  set  deeper 
than  it  was  in  its  original  location. 


FIG.  114.  The  roots  are  wrapped  to 
keep  them  from  drying,  and  tied  to  the 
trunk  of  the  tree.  The  natural  environ- 
ment of  the  roots  is  the  moist  soil,  and  it  is 
very  important  that  they  be  kept  moist ; 
even  a  few  minutes'  drying  may  be  fatal 
to  the  delicate  rootlets. 


FIG.  115.  The  tree  is  placed  on  a  sled 
and  securely  fastened.  It  is  then  hauled 
to  the  new  location,  where  the  roots  are 
carefully  spread  out  and  covered  with 
soil.  Fertile  topsoil  and  not  the  raw 
subsoil  should  be  used  for  covering  the 
roots. 


Roots  and  Their  Environment 


193 


FIG.  1 1 6.  The  newly  planted  tree  is 
supported  by  ropes  or  wires  until  the 
roots  can  take'  hold  and  anchor  it 
securely.  The  soil  should  be  packed 
firmly  about  the  roots  to  facilitate  the 
absorption  of  water  and  allow  the  quick 
development  of  rootlets  and  root  hairs. 


FIG.  117.  The  tree  must  be  kept  well 
watered  (but  not  flooded)  for  a  time,  for 
many  of  the  absorbing  rootlets  have  been 
lost  and  there  is  danger  that  the  leaves 
will  not  receive  an  adequate  water  sup- 
ply. By  this  method  large  trees  are  suc- 
cessfully transplanted  when  in  full  leaf. 


Success  in  transplanting  is  attained  by  gradually  trimming 
the  roots  months  before  the  tree  is  moved,  and  by  loosening 
the  soil  near  the  tree  so  as  to  develop  a  mass  of  absorbing 
roots  near  the  base  of  the  stem.  When  the  tree  is  lifted,  the 
roots  are  not  cut  off,  but  as  many  as  possible  of  them  are 
carefully  removed  from  the  soil.  The  small  roots  of  trees  are 
killed  by  drying,  and  for  this  reason  they  are  protected  from 
wilting  by  being  bound  up  in  wet  moss.  Sometimes  the  trees 
are  loosened  somewhat  in  the  autumn  and  moved  during  the 
winter,  together  with  much  of  the  frozen  soil  surrounding 
the  roots.  Successful  transplanting  depends  upon  reducing 
temporarily  the  loss  of  water  by  trimming  the  top,  preserving 
the  absorbing  roots,  and  exercising  care  in  handling  both 
roots  and  stems  so  that  they  may  not  be  injured.  (See  also 
Page  55.) 


194 


Science  of  Plant  Life 


Roots  in  relation  to  bacteria  and  fungi.     The  roots  of  many 
plants  have  bacteria  or  fungi  growing  about  them  or  inside 

them.  The  best-known  crop 
plants  belonging  to  this  group 
are  the  clover,  cowpea,  and 
alfalfa;  their  roots  develop 
small  nodules  in  which  cer- 
tain kinds  of  bacteria  change 
nitrogen  of  the  air  into  nitro- 
gen compounds  which  may 
be  used  by  the  plants.  More 
information  about  these  bac- 
teria will  be  found  in  a  later 
chapter  (page  258). 

Many  of  our  trees  and 
shrubs  have  fungi  surround- 
ing their  roots.  The  beech 

FIG.  118.    Roots  of  soy  bean,  showing  nodules    tree     for   example,   flourishes 
containing  nitrogen-fixing  bacteria. 

only  when   it  grows  under 

such  conditions.  The  difficulty  in  transplanting  azaleas, 
laurels,  and  rhododendrons  from  the  woods  to  our  lawns  lies 
largely  in  supplying  conditions  favorable  to  the  fungi  that 
invest  the  roots.  It  is  easy  to  supply  the  proper  shade  and 
water  conditions  for  the  shrubs,  but  it  is  difficult  to  furnish 
soil  conditions  favorable  to  the  life  of  the  fungi.  The  trans- 
planting of  these  shrubs  is  therefore  most  frequently  success- 
ful when  they  are  planted  in  large  bodies  of  soil  brought  with 
them  from  their  natural  habitat.  Just  how  the  fungi  aid  the 
plant  is  not  understood ;  that  they  are  essential  is  very  clear. 
Commercial  uses  of  roots.  The  fleshy  roots  of  the  sweet 
potato,  yam,  turnip,  carrot,  beet,  celeriac,  and  salsify  are 


Roots  and  Their  Environment  195 

important  sources  of  food.  The  sugar  beet  is  the  most  im- 
portant domestic  source  of  sugar  in  the  United  States.  The 
roots  of  sassafras,  rhubarb,  ginseng,  aconite,  and  ipecac  are 
used  in  medicine.  In  the  past  many  other  roots  were  collected 
for  the  same  purpose.  The  roots  of  plants  are  used  by  man 
much  less  than  are  stems  and  leaves.  Perhaps  roots  are  in 
general  less  useful ;  but  the  failure  to  utilize  them  more  gen- 
erally is  in  part  accounted  for  by  the  difficulty  of  harvesting 
them  and  by  our  lack  of  knowledge  concerning  uses  that 
might  be  made  of  them. 

PROBLEMS 

1.  What  states  produce  sugar  from  the  sugar  beet? 

2.  What  states  produce  sweet  potatoes? 

3.  How  are  roots  made  use  of  in  the  jetties  at  the  mouth  of  the  Mississippi 
River? 

4.  Why  do  trees  along  city  streets  frequently  die  when  the  street  is  paved  or 
cement  walks  are  laid  inside  the  curb  ? 

5.  Why  does  the  filling  in  of  wooded  land  with  a  3  or  4  foot  layer  of  soil  kill 
trees? 

6.  What  flood-plain  trees  will  continue  to  live  in  an  area  that  is  flooded  with 
water  for  a  year  or  more,  as  by  the  building  of  a  dam?     Why?    Why  do 
other  trees  die? 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Sixteen 

1.  Examine  and  make  diagrams  of  several  types  of  flower 
clusters,  such  as  spike,  catkin,  umbel,  raceme,  and  panicle. 

2.  Draw   several    flowers    of    different   types.     Label   pistil, 
stamen,  corolla,  calyx,  petal,  sepal,  receptacle,  peduncle,  anther, 
filament,    ovulary,    style,  and  stigma.     Note  differences  between 
monocot  and  dicot  flowers. 

3.  Examine  pollen  under  a  microscope,  and  germinate  grains 
in  sugar  solutions.     Try  various  strengths  from  5  to  10  per  cent. 
Place  pollen  in  a  drop  of  the  solution  on  a  micro  slide,  cover  with 
cover  glass,  and  keep  in  a  moist  chamber.     The  pollen  tubes  develop 
by  the  following  day. 

4.  Examine  the  stigmas  of  flowers  for  pollen  that  has  ger- 
minated. 

5.  Soak  different  kinds  of  seeds  in  water  and  study  them, 
noting  embryo,  endosperm,  seed  coats,  cotyledons,  hypocotyl,  and 
plumule. 

6.  Study  mature  pine  cones  with  seeds  in  place,  noting  the 
number  of  the  seeds  and  their  relation  to  the  scales.     Dissect  a 
seed  and  note  the  seed  coats,  endosperm,  and  embryo. 

7.  Germinate  some  seeds  and  follow  the  stages  in  germination. 

8.  Collect  as  many  kinds  of  fruits  as  possible.     Study  them 
from  the  standpoint  of  their  contents,  their  origin  from  the  flower, 
and  their  methods  of  dispersal.     Draw  as  many  as  convenient. 

9.  Field  trip  to  study  the  variety  of  flowers  and  flower  clusters. 

10.  Methods  of  pollination  may  best  be  studied  in  the  field. 
Note  the  kinds  of  insects  that  visit  a  particular  kind  of  flower. 
Compare  these  insects  with  those  that  visit  another  kind  of  flower. 
Is  there  a  noticeable  difference  in  the  forms  of  the  two  flowers? 
in  the  location  of  the  pollen  ?  in  the  location  of  the  nectar  ?     Does 
this  account  for  the  difference  in  the  kinds  of  insect  visitors  ? 

11.  Examine  several  varieties  of  flowers  and  note  whether  or 
not  the  stamens  shed  their  pollen  when  the  stigma  is  ripe,  before  it 
is  ripe,  or  after.     Does  the  relative  time  at  which  the  stigma  and 
pollen  mature  favor  self -pollination,  insure  it,  make  it  difficult,  or 
prevent  it? 

196 


CHAPTER   SIXTEEN 

REPRODUCTION  IN  FLOWERING  PLANTS:   FLOWERS,  FRUITS, 

AND    SEEDS 


FIG.  IIQ.    The  plants  in  flower  have  reached  the  reproductive  phase  of 
their  life. 

THE  development  of  roots,  stems,  and  leaves  makes  up  the 
first  period  of  the  life  of  a  flowering  plant.  These  parts  form 
the  vegetative  body  of  the  plant,  and  are  all  primarily  con- 
cerned, as  we  have  seen,  with  the  production,  distribution, 
and  accumulation  of  food  materials.  The  period  of  growth  of 
the  vegetative  parts  and  of  activity  in  connection  with  the 
food  supply  is  the  nutritive  phase  of  the  plant's  life. 

The  development  of  flowers,  fruits,  and  seeds  takes  place 
during  the  second  phase  of  the  plant's  existence,  the  repro- 
ductive phase.  The  foods  that  have  previously  accumulated 
in  various  parts  of  the  plant  are,  to  a  large  extent,  transferred  ; 
they  are  used  in  the  building  of  the  reproductive  structure? 

197 


198  Science  of  Plant  Life 

and  in  supplying  the  seeds  with  the  store  of  nourishment 
needed  for  germination  and  for  the  early  growth  of  the  seed- 
lings. The  reproductive  phase  of  the  plant's  life  is  essentially 
a  food-transferring  and  food-consuming  one ;  it  begins  with 
the  production  of  the  flower  and  ends  with  the  maturing  of 
the  seeds.  In  perennials  these  phases  are  not  distinct  as  they 
are  in  annuals  and  biennials. 

The  flower.  The  flower  fe  a  specialized  shoot,  whose  end 
is  the  production  of  seed.  Commonly  the  word  "  flower  " 
is  associated  with  the  brightly  colored  parts  that  make  many 
of  our  garden  and  house  plants  so  attractive.  But  here  we 
shall  include  under  the  term  the  simple  structures  associated 
with  seed  production  in  plants  like  the  grasses,  poplars,  and 
birches,  that  have  merely  scalelike  leaves  and  bracts  inclosing 
the  reproductive  parts.  In  the  conifers  the  seeds  are  pro- 
duced on  scale  leaves  arranged  spirally  in  cones.  These  cones 
may  be  looked  upon  as  a  lower  type  of  flower,  structurally 
very  different  from  the  flowers  of  the  monocots  and  dicots. 

Flower  clusters.  The  arrangements  of  flowers  on  stems 
are  so  varied  in  different  plants  that  it  is  quite  beyond  the 
scope  of  this  book  to  describe  the  many  kinds  of  flower  clus- 
ters. In  many  plants  the  flowers  occur  singly  at  the  ends  of 
stems  or  lateral  branches,  as  in  the  tulip  and  in  some  varieties 
of  roses.  In  other  plants  they  are  arranged  in  groups,  as  in 
the  spike  of  the  common  plantain  and  cat-tail ;  the  catkin  of 
the  willow,  alder,  and  oak;  the  umbel  of  the  carrot,  onion, 
and  milkweed ;  the  raceme  of  the  snapdragon,  spring  beauty, 
black  locust,  and  larkspur;  and  the  composite  head  of  the 
sunflower  and  chrysanthemum.  The  head  of  wheat  and  the 
ear  and  tassel  of  corn  are  other  forms  of  flower  clusters.  Plants 
like  the  yucca,  curly  dock,  rhubarb,  broom  corn,  and  hy- 


Reproduction  in  Flowering  Plants  199 

drangea  furnish  examples  of  large  and  much-branched  flower 
clusters.  A  stem  which  bears  a  single  flower  or  flower  cluster 
is  the  flower  stalk  or  peduncle.  The  branches  of  the  peduncle 


FIG.  120.  Flowers  of  the  corn  plant.  The  panicle  of  staminate  flowers  (tassel)  is 
shown  above.  Below  are  the  pistillate  flowers  arranged  in  a  spike  (ear)  inclosed  by 
sheathing  leaves.  The  only  part  of  the  pistillate  flowers  exposed  to  the  air  is  the  long 
style  (silk). 


200  Science  of  Plant  Life 

which  bear  the  individual  flowers  in  a  flower  cluster  are  called 
pedicels. 

The  parts  of  the  flower.  The  apex  of  the  flower  stalk  is 
called  the  receptacle.  It  is  often  enlarged  and  serves  as  a 
place  of  attachment  of  the  various  floral  organs.  The  outer 
whorl  of  scales  or  leaflike  organs  is  the  calyx.  It  usually  is 
green  in  color,  and  in  the  bud  stage  it  completely  incloses  the 
flower.  The  individual  parts  of  the  calyx  are  called  sepals. 
Next  inside  the  calyx  is  a  whorl  of  white  or  brightly  colored 
leaves  that  make  up  the  corolla.  The  several  parts  of  the 
corolla  are  called  petals.  The  corolla  is  usually  the  attractively 
colored  part  of  the  flower  ;  but  in  some  flowers,  as  in  the  tulip 
and  clematis,  the  sepals  have  the  same  coloring  as  the  petals. 
The  calyx  and  corolla  are  often  spoken  of  as  the  floral  envelopes, 
because  in  the  bud  they  form  a  wrapping,  or  envelope,  for 
the  inner  parts  of  the  flower. 

Inside  the  corolla  is  a  group  of  stamens,  each  composed  of 
a  stalklike  filament  and  an  anther  that  contains  the  pollen. 
The  center  of  the  flower  is  occupied  by  one  or  more  pistils, 
each  made  up  of  an  ovulary,  style,  and  stigma.  The  ovulary 
is  the  enlarged  part  of  the  pistil  that  contains  the  ovules, 
which  develop  into  the  seeds.  The  style  is  the  stalk  above 
the  ovulary  that  bears  at  its  summit  the  stigma.  The  stigma 
is  usually  an  enlarged  surface,  which  secretes  a  sticky,  sugary 
solution  in  which  the  pollen  grains  are  caught  and  germi- 
nated. The  pistils  and  stamens  are  called  the  "  essential  or- 
gans "  of  the  flower,  because  they  produce  the  ovules  and 
pollen  which  are  the  two  elements  necessary  for  the  production 
of  seed. 

The  variety  of  floral  structures.  The  above  is  a  descrip- 
tion of  a  typical  flower;  but  in  the  plant  world  we  find  an 


Reproduction  in  Flowering  Plants 


201 


almost  endless  variation  in  the  number,  form,  size,  color,  and 
arrangement  of  these  parts.  In  some  flowers  the  calyx  or 
the  corolla  may  have  their  parts  united  into  a  tube,  or  one  or 
both  may  be  wanting.  Or  the  flowers  may  lack  either  pis- 
tils or  stamens.  For  example,  the  soft  maples  bear  pistil- 
late flowers  on  some  trees  and  staminate  flowers  on  others, 
and  the  corn  has  staminate  flowers  in  the  tassel  and  pistil- 
late flowers  on  the  lower  lateral  branches  or  ears.  It  is  not 
our  purpose  to  name  and  describe  here  the  many  different 
variations  in  floral  structure;  a  visit  to  a  conservatory  or 
a  tramp  through  the  near-by  fields  and  woods  is  a  far  more 
effective  way  of  securing  an  idea  of  the  great  diversity  of 
flowers. 

Pollination.  If  the  stamens  of  a  lily  or  nasturtium  are 
examined,  the  pollen  is  found  to  be  a  fine  yellow  powder, 
which  under  a  microscope  will  be  seen  to  be  composed  of  a 
multitude  of  small  grains.  For  the  production  of  seed  it  is 
necessary  that  the  pollen  grains  shall  be  carried  to  the  stigma. 
This  transfer  is  called  pollination.  In  some  plants  the  pollen 


FIG.  121.     Flower  spikes  of  the  alder.    The  two  clusters  on  the  left  are  staminate 
spikes ;  on  the  right  the  mature  pistillate  spikes  are  shown. 


202  Science  of  Plant  Life 

merely  falls  by  gravity  on  the  stigma.  Wheat  and  oats  are 
examples  of  plants  that  are  pollinated  in  this  way.  In  other 
plants,  like  the  pines,  elms,  birches,  oaks,  rye,  and  corn,  the 
pollen  is  carried  by  the  wind.  It  is  an  interesting  fact  that 
the  stigmas  of  wind-pollinated  flowers  are  usually  roughened 
by  hairs,  which  probably  make  them  more  effective  in  hold- 
ing the  pollen. 

In  the  case  of  most  plants  with  conspicuous  flowers,  the 
pollen  is  carried  by  bees,  flies,  butterflies,  and  moths.  As 
the  body  of  one  of  these  insects  is  rough  or  hairy,  pollen  grains 
become  attached  to  it  when  the  insect  enters  a  flower.  Then 
when  the  insect  passes  to  another  flower,  some  of  the  pollen 
from  the  first  flower  is  brushed  off  on  the  stigma  of  the  second. 
Thus  pollination  is  brought  about  by  the  insects  in  the  course 
of  their  visits  to  successive  flowers.  It  is  an  advantage  to 
the  plant  to  have  its  pollen  carried  by  insects  directly  from 
flower  to  flower  instead  of  being  blown  about  and  reaching 
a  stigma  by  mere  chance.  If  the  amounts  of  pollen  pro- 
duced by  the  pine,  corn,  ragweed,  and  other  wind-pollinated 
plants  are  compared  with  the  amounts  produced  by  plants 
that  are  pollinated  by  insects,  it  will  be  seen  that  insect- 
pollinated  plants  generally  produce  less  pollen. 

Why  insects  visit  flowers.  Insects  do  not  visit  flowers  to 
carry  pollen  for  the  plants.  They  eat  the  pollen  or  feed  their 
young  on  it,  and  they  also  secure  nectar  from  the  flowers. 
The  nectar  is  a  watery  solution  containing  sugar,  which  is 
secreted  by  glands  called  nectaries.  One  or  more  of  these 
nectaries  is  usually  located  near  the  base  of  the  corolla,  in- 
side the  flower.  The  insects  visit  the  flowers,  therefore,  to 
secure  food  for  themselves,  but  as  they  make  their  visits  they 
brush  against  the  stigmas  and  leave  grains  of  pollen  adhering 


Reproduction  in  Flowering  Plants  203 

to  the  plants,  and  in  this  way  perform  a  service  for  the  plants. 
The  perfumes  of  flowers  aid  the  insects  in  finding  them,  and 
conspicuous  white  or  brightly  colored  parts  of  flowers  may 
serve  the  same  purpose.  The  massing  of  many  small  flowers 
in  clusters  and  heads  certainly  makes  them  more  conspicuous. 

Cross-pollination.  When  a  flower  is  pollinated  with  its 
own  pollen  or  with  that  from  another  flower  on  the  same 
plant,  it  is  said  to  be  self -pollinated.  If  the  pollen  comes  from 
another  plant,  a  flower  is  said  to  be  cross-pollinated.  In 
many  plants  it  makes  no  difference  whether  the  pollen  comes 
from  the  stamens  of  the  same  plant  or  from  those  of  another 
plant.  In  the  common  tobacco  plant  the  pollen  may  be 
transferred  to  the  stigma  of  the  same  flower,  and  seeds  will 
be  produced.  In  some  plants,  however,  it  is  only  when  the 
flowers  are  cross-pollinated  that  seeds  are  formed.  The 
sunflower  is  a  good  example  of  this  kind  of  plant.  In  still 
other  plants  seeds  that  are  formed  after  self-pollination  are 
less  vigorous  than  those  formed  after  cross-pollination. 

From  the  above  statements  it  will  be  seen  that  cross-pol- 
lination is  an  advantage  to  some  plants,  and  we  find  in  flowers 
many  arrangements  that  help  to  bring  this  about  and  to  pre- 
vent self-pollination.  Often  the  anthers  do  not  shed  their 
pollen  at  the  time  when  the  adjoining  stigma  is  in  condition 
to  receive  it.  The  pollen  may  be  shed  either  before  or  after 
the  ripening  of  the  stigma.  In  such  plants  there  is  little 
possibility  of  the  stigma's  being  pollinated  from  the  stamens 
of  the  same  flower.  So,  as  insects  go  from  one  flower  to 
another  they  transfer  pollen  from  flowers  in  which  the  pollen 
is  ripe  to  flowers  in  which  the  stigmas  are  ripe.  This  favors 
cross-pollination . 

It  is  exceedingly  interesting  to  study  the  various  other 


204  Science  of  Plant  Life 

mechanisms  that  favor  cross-pollination,  but  it  should  be  done 
in  the  field  or  with  the  flowers  in  hand.  In  the  white  lily  the 
stigma  is  out  of  reach  of  the  insects  when  the  pollen  is  shed. 
In  other  plants  the  pistillate  and  staminate  flowers  may 
occur  on  different  individuals,  or  on  different  branches  of  the 
same  plant.  In  primroses  and  bluets  the  stigmas  and 
stamens  each  have  two  different  lengths ;  the  flowers  on  one 
plant  have  long  styles  and  short  stamens,  while  the  flowers 
on  another  plant  have  short  styles  and  long  stamens. 

The  most  remarkable  cases  of  cross-pollination  by  insects 
are  those  in  which  a  particular  species  of  insect  is  necessary 
for  the  pollination  of  a  plant.  Such  relations  exist  in  the 
yuccas  and  in  some  orchids.  In  the  absence  of  the  particular 
insect,  pollination  and  seed  production  fail.  Yuccas  may  be 
grown  in  our  Northern  states,  but  in  certain  localities  they 
fail  to  produce  seeds  because  the  moth  (Pronuba)  needed  to 
pollinate  the  flowers  does  not  live  there. 

Formation  and  growth  of  the  pollen  tube.  A  second  step 
essential  to  the  production  of  seed  is  the  germination  of  the 
pollen  and  the  formation  of  the  pollen  tube.  After  a  grain  of 
pollen  is  placed  on  the  stigma,  a  microscopic  tube  develops 
from  its  side  and  grows  downward  among  the  cells  of  the 
stigma  and  style.  At  the  time  of  shedding,  the  pollen  grain 
of  most  flowering  plants  contains  three  cells.  One  of  the  three 
is  active  in  the  formation  of  the  pollen  tube ;  the  other  two  are 
the  sperms  or  male  cells. 

The  stigma,  as  we  have  seen,  secretes  a  sticky  fluid  con- 
taining sugar,  acids,  and  other  substances.  The  pollen 
germinates  best  in  the  fluid  secreted  by  the  stigmas  of  the 
same  kind  of  plant,  and  it  usually  germinates  imperfectly  or 
not  at  all  on  the  stigma  of  a  different  kind  of  plant.  Perfect 


Reproduction  in  Flowering  Plants 


L20S 


pollen  tubes  sometimes  develop,  however,  following  pollination 
from  related  species.     The  seeds  produced  by  such  crosses 


FIG.  122. 


Pollen  grains  and  pollen  tubes.    S  is  the  two  sperms  or  male  cells, 
and  T  the  tube  nucleus. 


often  give  new  forms  of  plants  that  are  different  from  both 
parents. 

After  germination  of  the  pollen,  the  pollen  tube  grows  down- 
ward through  the  stigma  and  style  to  the  ovule,  or  young 
seed.  Usually  this  is  but  a  short  distance.  In  corn,  how- 
ever, the  silk  is  the  style  and  stigma,  and  the  pollen  tube  must 
grow  several  inches  or  a  foot  down  the  silk  to  the  ovule  below. 
As  the  pollen  tube  lengthens,  the  sperms  pass  down  the  tube. 

Fertilization.  Inside  the  ovule  there  is  an  oval,  saclike 
body  which  contains  several  cells.  One  of  these  is  the  egg 
cell,  or  female  cell.  At  the  beginning  of  fertilization  the 
pollen  tube  grows  into  the  ovule  and  discharges  the  two 
sperms  into  the  sac  in  which  the  egg  cell  lies.  One  of  the 
sperms  unites  with  the  egg  cell.  This  union  of  the  sperm. 


206 


Science  of  Plant  Life 


or   male   cell,   with   the   egg,   or   female   cell,   is   the   act  oj 
fertilization. 

The  fertilized  egg  immediately  begins  to  develop ;  that  is,  it 
grows  and  divides  into  a  large  number  of  cells,  and  from  it 
eventually  comes  the  embryo  or  young  plant  that  is  found 
within  the  seed.  Fertilization  is  the  essential  part  of  sexual 
reproduction  both  in  plants  and  animals,  and  it  marks  the 
actual  beginning  of  a  new  generation.  After  the  egg  is  fer- 
tilized, it  is  able  to  develop  into  a  new  plant  or  animal  of  the 
same  kind  as  its  parents.  It  should  be  understood  that  the 
sperms  from  one  pollen  tube  fertilize  the  egg  in  only  one  ovule, 
and  that  to  fertilize  all  the  eggs,  as  many  pollen  tubes  must 


FIG.  123.  Diagram  to  illustrate  stages  in  the  reproduction  of  a  seed  plant.  A  shows 
pollen  grains  (b)  from  anther  (a)  on  top  of  stigma ;  B  shows  a  stage  in  the  growth  of 
the  pollen  tube  and  the  formation  of  the  egg  cell ;  C  is  the  stage  when  the  pollen  tube 
has  reached  the  egg  and  sperm  and  egg  unite;  D  shows  the  formation  of  the  embryo 
plant  within  the  seed;  and  E  shows  the  further  development  of  the  embryo  and  the 
maturing  of  the  seed  coats.  The  young  plant  thus  formed  and  incased  may  remain 
alive  for  years  within  the  seed. 


Reproduction  in  Flowering  Plants  207 

grow  down  through  the  style  as  there  are  ovules  in  the  ovulary 
below. 

The  seed.  The  seed  is  the  final  product  of  pollination  and 
fertilization.  Its  complete  development  ends  the  role  of  the 
flower.  The  essential  part  of  the  seed  is  the  embryo,  for  it 
is  the  embryo  that  will  later  produce  the  seedling;  and  in 
order  to  supply  the  embryo  with  nourishment  during  the 
early  stages  of  growth,  food  from  the  parent  plant  is  ac- 
cumulated in  the  seed.  This  may  either  be  stored  inside  the 
embryo  itself,  as  in  the  bean ;  or  it  may  be  stored  in  the  tissue 
surrounding  the  embryo,  as  in  the  castor  bean  and  corn.  The 
food-containing  tissue  surrounding  the  embryo  in  many  seeds 
is  called  the  endosperm.  Since  the  seed  carries  the  plant  over 
the  winter  or  through  an  unfavorable  season,  the  embryo  with 
its  food  supply  is  protected  by  the  seed  coats,  one  of  which  is 
usually  hard  and  resistant. 

The  three  constituents  of  a  seed,  then,  are  (i)  the  embryo 
or  young  plant,  (2)  the  food  supply,  either  inside  the  embryo 
or  in  the  endosperm,  and  (3)  the  seed  coats  or  protective  cover- 
ing. 

The  structure  of  seeds.  Although  seeds  vary  as  much  in 
form  as  do  other  plant  organs,  the  different  arrangements  of 
the  three  essential  parts  may  be  illustrated  by  a  castor  bean, 
a  bean,  and  a  grain  of  corn. 

In  the  castor  bean  the  seed  coats  consist  of  a  hard  outer 
layer  and  a  thin  inner  membrane.  These  inclose  an  endo- 
sperm, which  is  a  mass  of  cells  containing  food  in  the  form  of 
starch,  oil,  and  protein.  Within  the  endosperm  lies  the 
embryo,  ready  to  grow  when  favorable  conditions  for  germi- 
nation come.  The  embryo  consists  of  the  hypocotyl  and  two 
cotyledons,  with  a  small  bud  between  the  cotyledons,  called 


208 


Science  of  Plant  Life 


the  plumule.  The  cotyledons  are  the  first  leaflike  organs  of 
the  plant.  The  hypocotyl  is  the  first  stem,  and  the  plumule 

is  the  first  bud.  No  root  is 
found  in  the  embryo ;  but 
when  the  seed  germinates  the 
hypocotyl  elongates,  and  from 
its  end  the  primary  root  de- 
velops. The  cotyledons  at 
first  absorb  food  from  the  en- 
dosperm and  later  expand  into 
photosynthetic  organs,  which 
become  green  when  exposed 
to  the  light.  The  plumule 
grows  upward  to  form  the 
stem.  All  these  early  changes 
are  made  at  the  expense  of  the 
food  in  the  endosperm. 

The  bean  seed  consists 
merely  of  the  embryo  with  a 
seed  coat  about  it.  The  food 
in  this  seed  has  already  been 

FIG.  124.    Development  of  mangrove  seed-  . 

lings.  This  small  tree  grows  on  soft  mud  absorbed  into  the  embryo  and 
flats  in  the  tropics  and  semi-tropics.  The  stored  in  the  greatly  thickened 

seed  (A  and  C)  germinates  while  still  at-  . 

tached  to  the  tree  and  forms  an  embryo  a  Cotyledons  ;    that  IS,  the  young 

foot  or   more   in    length.      The   embryo  embryo      has      Continued       its 

finally  drops  endwise  like  an  arrow  into  ,              _,,                 -,           ,  .1     •, 

the  mud  and  starts  a  seedling  (D).  growth     m    the    Sf;ed     Untl1    * 

has  all  the  food  inside  itself. 

The  parts  of  the  embryo  are  the  same  as  in  the  castor 
bean,  but  the  cotyledons  are  thick  and  contain  a  great 
supply  of  food  for  the  young  plant.  The  bean  is  an  ex- 
ample of  a  large  class  of  plants,  including  the  pea,  squash, 


Reproduction  in  Flowering  Plants 


209 


Bureau  of  Science,  P.  I. 

FIG.  125.  A  mangrove  swamp.  Note  the  prop  roots  which  support  the  trees, 
and  the  young  plants  in  the  foreground  which  have  dropped  from  the  overhanging 
branches. 

and  pumpkin,  in  the  mature  seeds  of  which  the  endosperm 
is  lacking. 

A  grain  of  corn  is  an  example  of  a  third  kind  of  seed.  In  it 
there  is  a  large  endosperm  with  a  small  embryo  placed  at  one 
end  of  it.  The  embryo  differs  from  the  embryos  of  the  bean 
and  the  castor  bean  in  that  it  has  only  a  single  cotyledon, 
wrapped  more  or  less  around  the  hypocotyl  and  plumule. 
In  germinating,  the  hypocotyl  grows  downward,  and  the 
primary  root  develops  from  its  tip.  The  plumule  grows 
upward  to  form  the  aerial  shoot.  As  in  the  castor  bean,  the 
cotyledon  is  the  absorbing  organ  through  which  the  foods  in 
the  endosperm  enter  the  young  plant. 

The  flower  and  embryo  in  monocots  and  dicots.  In  dis- 
cussing the  subject  of  stems,  attention  was  called  to  the  fact 


210 


Science  of  Plant  Life 


Bureau  of  Agriculture,  P.  I. 

FIG.  126.     A  pineapple  field.     The  pineapple  fruit  is  an  enlarged  fleshy 
flower  cluster. 

that  flowering  plants  are  divided  into  two  great  groups,  the 
monocots  and  dicots.  The  monocots  have  parallel-veined 
leaves ;  the  bundles  of  the  stem  are  closed  (have  no  cambium) ; 
and  the  bundles  are  not  arranged  in  a  circle.  The  dicots  in- 
clude forms  with  net- veined  leaves;  the  stem  bundles  are 
open  (have  a  cambium) ;  and  they  are  arranged  in  a  circle. 

The  terms  "  monocot  "  and  "  dicot  "  (or,  as  they  are  fre- 
quently written,  "  monocotyledon  "  and  "  dicotyledon  ")  are 
based  on  the  apparent  number  of  cotyledons  in  the  embryo, 
whether  there  are  one  or  two.  Any  one  who  has  watched 
plants  beginning  to  grow  in  a  garden  will  recall  the  two 
cotyledons  of  the  bean,  pumpkin,  sunflower,  and  radish,  raised 
above  the  soil.  It  will  also  be  recalled  that  these  plants 
have  net- veined  leaves.  The  cotyledon  of  a  monocot'  is 
usually  an  absorbing  organ  that  remains  below  the  ground  in 
contact  with  the  endosperm. 


Reproduction  in  Flowering  Plants 


211 


The  two  groups  differ  in  their 
flowers  also.  In  the  monocots  the 
number  of  parts  of  the  calyx  and 
corolla  is  usually  three,  and  the 
stamens  and  divisions  of  the  pistil 
are  three  or  some  multiple  of  three. 
In  the  dicots  the  parts  of  the  flower 
are  typically  in  fives  or  fours,  or  in 
a  multiple  of  these. 

Thus  the  names  "monocot"  and 
"  dicot  "  relate  to  the  form  of  the 
embryo ;  but  the  two  groups  are 
further  distinguished  by  differences 
in  leaf  venation,  bundle  structure, 
bundle  arrangement,  and  flower 
plan. 

The  gymnosperms  and  angio- 
sperms.  We  have  previously  learned 
that  the  conifers  bear  their  seeds  on 
scale  leaves  arranged  in  cones.  A 
study  of  one  of  these  cones  shows 
that  the  seeds  are  formed  on  the 
upper  surfaces  of  the  scales  and  are 
not  inclosed  in  capsules.  When  the  scales  mature  and  be- 
come dry,  the  cone  opens  and  the  seeds  fall  out.  The  word 
"  gymnosperm  "  means  "  naked  seed,"  and  this  is  the  group 
name  for  the  conifers  and  all  other  plants  whose  seeds  are 
not  inclosed. 

The  angiosperms  are  what  we  usually  call  the  flowering 
plants,  although  some  of  them,  like  the  grasses  and  forest 
trees,  do  not  produce  flowers  with  colored  parts.  The  seeds 


U.  S.  Dept.  of  Agriculture 
FIG.  127.  Fruit  of  mango. 
This  much-prized  fruit  has 
hitherto  been  grown  only  in 
the  tropics,  but  recently  it  has 
been  introduced  into  southern 
Florida. 


212 


Science  of  Plant  Life 


U.  S.  Dept.  of  Agriculture 

FIG.  128.  Avocado,  or  alligator 
pear,  a  salad  fruit  now  being  grown, 
in  southern  Florida  and  California. 


U.  S.  Dept.  of  Agriculture 

FIG.  1 29.  Raspberries,  a  fruit 
which  is  made  up  of  a  collection 
of  fleshy  pistils. 


of  an  angiosperm,  in  contrast  to  those  of  a  gymnosperm,  are 
inclosed  in  an  ovulary  commonly  called  a  pod  or  capsule,  as 
in  the  bean,  horse-chestnut,  hickory  nut,  and  watermelon. 
The  term  "  angiosperm  "  means  "  hidden  or  covered  seed," 
referring  to  the  fact  that  the  seeds  are  inclosed  by  a  pod, 
fleshy  fruit,  or  other  covering. 

The  fruit.  The  term  "fruit"  is  commonly  used  to  desig- 
nate a  great  variety  of  organs  that  are  developed  as  a  result 
of  the  flowering  and  pollination  of  plants.  The  direct  result 
of  pollination  and  fertilization  is  the  production  of  the  seed. 
The  indirect  effect  of  pollination  is  the  further  development  of 
adjacent  structures.  Primarily  the  fruit  is  the  enlarged  pistil 
or  ovulary ;  but  in  many  cases  the  calyx  and  the  receptacle  also 
enlarge  and  form  a  part  of  the  fruit  —  sometimes  most  of  it. 


Reproduction  in  Flowering  Plants 


2*3 


In  the  cucumber,  watermelon,  yucca,  and  tomato  the 
fruit  is  the  enlarged  ovulary.  In  the  okra,  wild  geranium, 
maple,  hickory,  and  elm  the  entire  pistil  is  involved.  In  the 
haw,  apple,  blueberry,  and  pear  the  calyx  forms  part  of  the 
fruit.  In  the  strawberry,  lotus,  rose,  and  fig  the  fleshy  part 
of  the  fruit  is  the  enlarged  receptacle.  It  is  a  remarkable  fact 
that  the  fertilization  of  the  egg  in  an  ovule  not  only  causes 
the  egg  to  develop  into  a  new  plant  (the  embryo),  but  also 
causes  other  parts  of  the  flower  to  enlarge  and  become  fruit. 

The  grains  like  wheat  and  corn  are  both  seeds  and  fruits, 
for  the  outer  coat  contains  the  ovulary  wall  as  well  as  the 
seed  coats  proper. 

Economic  importance  of  flowers,  fruits,  and  seeds.  The 
economic  value  of  flowers  lies  chiefly  in  their  use  for  decorative 
purposes,  but  certain  flower  clusters  like  the  artichoke  and 
cauliflower  are  used  as  food.  The  fruit  industry  needs  only 
to  be  mentioned  to  call  to  mind  the  vast  scale  upon  which 
plants  are  grown  for  their  fleshy  edible  fruits.  It  should  be 


U.  S.  Dept.  of  Agriculture 
FIG.  130.     Coffee  berries,  natural  size.    Each  contains  two  seeds. 


214 


Science  of  Plant  Life 


noted  that  ripe  fruits  are  made 
up  largely  of  water  pleasantly 
flavored  with  sugar,  dilute 
acids,  and  aromatic  sub- 
stances. The  amount  of  food 
actually  present  is  usually 
small.  Fruits  are  valuable 
chiefly  for  the  variety  which 
they  add  to  our  diet.  Through 
canning,  preserving,  and  dry- 
ing they  are  made  available  at 
all  seasons  of  the  year. 

Seeds  and  grains  supply  the 
most  concentrated  foods  de- 
rived from  plants.  They  pro- 
vide the  larger  part  of  the  food 
of  all  human  beings.  Seeds  of 
cotton,  corn,  and  coconut  fur- 
nish enormous  quantities  of 
oils  used  in  the  manufacture  of 
various  fats  and  soap,  and  nuts  of  various  kinds  are  coming 
to  be  used  more  and  more  extensively  as  foods.  Corn  oil  is 
widely  used  in  the  making  of  rubber.  Flaxseed  is  the  source 
of  linseed  oil,  which  is  used  in  the  manufacture  of  paints. 
From  the  grains  we  derive  also  starch,  glucose,  alcohol, 
ether,  and  many  related  organic  substances.  The  seeds  of 
the  coffee  and  cacao  plants  supply  pleasant  and  mildly 
stimulating  drinks.  The  hairy  covering  of  the  cottonseed  is 
the  most  important  fiber  used  in  the  manufacture  of  cloth. 


U.  S.  Dept.  of  Agriculture 
FIG.  131.  Persimmon  fruits.  The 
persimmon  grows  wild  over  a  large 
part  of  the  Southeastern  states,  and 
improved  varieties  are  now  cultivated. 


Reproduction  in  Flowering  Plants 


215 


Bureau  of  Agriculture,  P.  I . 

FIG.  132.  Drying  copra,  the  fleshy  portion  (endosperm)  of  the  coconut  seed.  The 
coconuts  are  split  and  set  in  the  sun,  and  the  meats,  after  they  shrink  and  become 
loose,  are  removed  from  the  shells  and  placed  on  mats  to  dry.  Copra  is  the  source  of 
coconut  oil. 

PROBLEMS 

1.  Why  does  a  corn  plant  growing  alone  produce  imperfect  ears? 

2.  When  cucumbers  are  grown  in  commercial  greenhouses,  how  is  pollination 
accomplished  ? 

3.  Why  are  large,  heavy  seeds  of  agricultural  plants  more  desirable  for  plant- 
ing than  small,  light  seeds? 

4.  What  common  market  "vegetables"  are  included  in  the  botanical  term 
"fruit"? 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Seventeen 

1.  Review  and  as  far  as  possible  illustrate  with  specimens  tne 
various  methods  of  propagating  plants  vegetatively.     Here  may 
be  included : 

a.  Bulbs,  tubers,  corms,  and  rootstocks. 

b.  Cuttings,  as  in  the  geranium  and  grape. 

c.  Runners  and  horizontal  stems,  as  in  strawberry  and  blue 

grass. 

d.  Leaf  cuttings,  as  in  Begonia  and  Bryophyllum. 

e.  Small  lateral  branches  (suckers),  as  in  carnation,  pine- 

apple, and  banana. 

2.  Make  a  field  trip  to  a  commercial  greenhouse  or  nursery  to 
see  how  plants  are  propagated. 

3.  Report  on  the  origins  of  some  of  the  common  garden  vege- 
tables and  field  crops.     Information  may  be  obtained  in  encyclo- 
pedias and  in  agricultural  and  horticultural  textbooks. 

4.  Study  a  few  weeds  in  the  field,  trying  in  each  case  to  deter- 
mine why  the  plant  is  successful  in  growing  where  it  is  not  wanted. 
List  the  weeds,  and  put  the  results  of  your  study  in  the  form  of  a 
table : 


NAME  OF  WEED 

PROPAGATED  BY 

REPRODUCED  BY 

SEEDS  CARRIED  BY 

WHY  DIFFICULT 
TO  EXTERMINATE 

5.  What  are  the  five  commonest  weeds  of  lawns?  of  cornfields? 
of  pastures?  of  gardens?  Why  do  different  kinds  of  weeds  occur 
in  these  several  habitats  ? 


216 


CHAPTER   SEVENTEEN 

REPRODUCTION   IN   RELATION   TO   AGRICULTURE 

THE  reproduction  of  plants  is  of  particular  interest  in  agri- 
culture because  the  reproductive  structures  themselves  have 
an  economic  value.  In  a  former  chapter  the  importance  of 
seeds  and  fruits  as  sources  of  food  has  been  discussed.  In  this 
chapter  we  shall  consider  the  relation  of  reproduction  to  three 
important  phases  of  agriculture:  plant  propagation,  plant 
breeding,  and  weed  control.  Propagation  and  breeding  fur- 
nish better  crop  plants ;  weed  control  provides  more  pro- 
ductive fields. 

Vegetative  multiplication.  In  the  discussion  of  stems 
(page  159)  attention  was  called  to  the  fact  that  one  of  the 
advantages  in  underground  stems  lies  in  the  facility  with 
which  the  plant  may  be  multiplied.  From  rootstocks  arise 
new  terminal  and  lateral  buds  that  later  form  new  aerial 
shoots,  and  through  the  death  of  the  older  parts  of  the  under- 
ground stems  these  branches  become  separate  plants.  Bulbs, 


FIG.  133.     Multiplication  of  the  raspberry.     The  stems  arch  over  and  take  root 

where  they  touch  the  ground,  thus  starting  new  plants. 

217 


218  Science  of  Plant  Life 

corms,  and  tubers  bring  about  vegetative  propagation  in  a 

similar  way. 

Plants  may  multiply  from  the  aerial  vegetative  parts  also. 

The  stems  of  the  raspberry  com- 
monly bend  over,  and  where  they 
touch  the  ground  they  form  buds 
from  which  adventitious  roots  and 
new  upright  stems  develop.  A 
grapevine  will  take  root  where  a 
node  comes  in  contact  with  the 
soil.  In  the  walking  fern  (page 

FIG.  134.  Bryophyllum  leaf,  with  283)  the  tips  of  the  leaves  develop 
young  plants  starting  from  the  - 

notches  in  the  margin.  buds,  roots,  and  new  plants  when 

in  contact  with  the  soil.  The 

strawberry  is  an  example  of  certain  plants,  including  many 
grasses,  which  have  horizontal  branches  (runners)  on  the  soil 
surface  that  take  root  at  intervals  and  produce  new  plants. 
In  Bryophyllum,  a  weed  in  cultivated  fields  of  the  West 
Indies,  the  leaves  when  they  fall  to  the  ground  develop  new 
plants  from  the  notches  in  their  margins  (Fig.  134). 

These  illustrations,  which  might  be  multiplied,  show  the  im- 
portance of  vegetative  propagation  in  the  increase  and  spread 
of  plants.  Among  wild  plants  it  is  entirely  possible  that  vege- 
tative multiplication  is  as  effective  in  spreading  the  plants  as 
is  reproduction  by  seeds.  By  the  former  method  the  young 
plant  is  able  to  start  more  vigorously  than  a  seedling,  because 
it  is  able  to  draw  water  and  food  materials  from  the  parent 
plant  until  its  own  root  and  leaf  systems  are  well  developed. 

Vegetative  propagation  of  cultivated  plants.  In  agriculture 
and  horticulture,  vegetative  multiplication  is  relied  upon  for 
starting  many  cultivated  plants.  Potatoes,  mint,  horse- 
radish, sugar  cane,  sweet  potatoes,  and  certain  varieties  of 


Reproduction  in  Relation  to  Agriculture        219 


Bureau  of  Science,  P.  /. 

FIG.  135.    Sugar-cane  cuttings.    The  plants  are  started  in  the  fields 
from  cuttings  and  not  from  seeds. 

onion  are  examples  of  crop  plants  started  in  this  way.  Gera- 
niums, coleus,  willows,  currants,  grapes,  and  most  ornamental 
shrubs  are  grown  from  cuttings.  These  cuttings  are  pieces 
of  stem  containing  two  or  more  nodes.  Propagation  by 
bulbs,  conns,  or  rootstocks  is  the  method  commonly  employed 
in  starting  lilies,  tulips,  hyacinths,  irises,  cannas,  Caladiums, 
and  chrysanthemums.  Many  of  our  fruit  trees  are  multi- 
plied by  budding  and  grafting,  which  are  specialized  forms 
of  vegetative  propagation  (page  132). 

Vegetative  propagation  has  been  found  advantageous  in 
crop  plants  wherever  its  use  is  possible,  (i)  because  the  vari- 
ability of  the  plants  produced  is  much  less  than  when  they  are 
propagated  by  seeds,  (2)  because  some  plants,  like  the  sugar 
cane,  banana,  and  horseradish,  do  not  produce  seed,  and  (3)  be- 
cause it  saves  time  in  securing  the  product,  as  a  longer  period 
is  required  for  the  maturing  of  plants  started  from  seeds.  In 
practical  plant  production,  vegetative  multiplication  is  as 
important  as  reproduction  by  seeds. 


220 


Science  of  Plant  Life 


Plant   breeding.      Reproduction   in   plants    is    the 
foundation  upon  which  the  important  industry  of  plant 
breeding  is  being  built.     Plant  breeding  is  concerned 
with  the  improvement  of   economic  plants  and  with 
the  production  of  new  plants  of  economic  value.     The 
activities  of  plant  breeders  are 
being     directed     toward     four 
principal     objectives :     (i)    the 
breeding   of   plants   with    more 
desirable   products,    as   flowers, 
fruits,  leaves,  and  fibers ;  (2)  the 
breeding  of  new  varieties  which 
will  increase  the  yield  per  acre ; 
(3)    the    securing    of    varieties 
better   fitted   to   particular  cli- 
mates and  soils ;    and    (4)    the 
producing  of  varieties  capable  of 
greater  resistance  to  diseases. 

Plant  breeding  with  these  pur- 
poses is  possible  and  profitable 
because  (i)  variations  naturally 
occur  among  plants ;  (2)  some 
variations  are  inherited  and 
may  be  preserved  by  selection; 
and  (3)  different  varieties  and 
species  may  be  crossed  to  pro- 
duce hybrids  having  a  still  larger 
range  of  variations  than  the 

number    of    seeds    produced,    and    by    parent  plants  Or  possessing  new 

selecting    seed    from   only    the   plants     combinations  of   desirable  quali- 

with  long  spikes  the  yield  of  seed  may 

be  increased  several  fold.  tlCS. 


FIG.  136.    Timothy  spikes.    The  heads 
show   great  variation    in    length    and 


Reproduction  in  Relation  to  Agriculture        221 


Bureau  of  A  griculture,  P.  /. 

FIG.  137.  Four  types  of  Indian  corn.  From  left  to  right :  Sweet  corn,  which  has  a 
high  sugar  content  and  shrivels  when  dried ;  dent  corn,  which  has  a  depression  in 
the  crown  of  each  kernel  due  to  the  shrinking  of  the  endosperm ;  flint  corn,  which  has 
a  hard  endosperm  and  rounded  kernels;  and  popcorn,  which  is  a  small,  extremely 
flinty  variety. 

The  methods  used  by  the  plant  breeder  depend  upon  the 
reproductive  structures  and  habits  of  the  particular  plants 
with  which  he  is  working.  For  example,  the  means  by  which 
a  plant  is  naturally  pollinated  will  determine  how  it  must  be 
handled  at  the  time  of  flowering  to  secure  self-pollinated  seed 
or  cross-pollinated  seed.  Methods  of  vegetative  propagation 
may  be  used  to  multiply  the  plant  after  a  desirable  variety 
has  been  produced.  In  this  way  the  plant  grower  avoids 
cross-pollination  and  the  variations  that  appear  when  many 
cultivated  crops  are  grown  from  seed.  The  best  ways  of 
propagating  particular  crops  vegetatively  may  be  found 


222 


Science  of  Plant  Life 


described  in  recent  publications  devoted  to  plant  production 
and  plant  breeding. 

Variations.  No  two  fruits,  flowers,  or  other  plant  organs 
are  exactly  alike.  The  variations  may  be  small  or  large,  and 
there  may  be  every  gradation  between  the  extremes  of  any 
character.  The  several  thousand  sunflowers  that  might  be 
grown  from  a  pound  of  seed  would  vary  in  height  of  stem, 
amount  of  branching,  and  size  of  flowers.  Not  only  may 
there  be  variations  in  the  structures  of  plants,  but  there  may 
also  be  variations  in  the  composition  of  the  plant  organs. 
For  example,  the  great  variety  of  colors,  flavors,  and  other 
qualities  in  apples  is  due  to  variations  in  the  composition  of 
this  fruit.  The  variation  in  each  of  these  characters  is  quite 
independent  of  variations  in  the  others.  It  is  possible,  there- 
fore, to  get  all  kinds  of 
combinations  of  different 
characters,  and  by  careful 
breeding  and  selection  to 
combine  many  desirable 
qualities  in  a  single  plant. 
The  Shasta  daisy,  for  ex- 
ample, was  made  by  breed- 
ing together  the  English, 
American,  and  Japanese 
daisies,  and  combining  in 
one  plant  the  pleasing  foli- 
age of  the  English  species, 

FIG.  138.    Two  plants  of  sweet  corn  of  the    the    free-blooming    habit    of 
same  variety,  one  grown  in  poor  soil  and     the  American  daisy,  and  the 

waxy  luster  of  the  petals  of 


one  in  soil  to  which  fertilizer  was  added. 
The  differences  in  the  plants  are  due  to  the 
environment. 


the  Japanese  plant. 


Reproduction  in  Relation  to  Agriculture        223 


Bureau  of  A  griculture,  P.  I. 

FIG.  139.     Five  varieties  of  lima  beans,  all  grown  in  the  same  soil.     The  differences 
are  due  to  differences  in  the  plants. 

Two  kinds  of  variations.  When  a  crop  plant  like  corn  is 
grown  in  rich  soil  and  in  poor  soil,  there  are  great  differences 
in  the  size  of  the  plants  and  in  the  yield  per  acre.  These 
variations  in  size  and  yield  are  due  to  the  environment.  Even 
though  the  seed  planted  in  each  kind  of  soil  is  exactly  the 
same,  there  will  be  a  wide  difference  in  the  plants. 

On  the  other  hand,  there  are  variations  that  are  due  to  in- 
nate differences  in  the  plants  themselves.  If  dwarf  sweet 
corn  and  large  field  corn  be  planted  in  the  same  soil  and 
given  the  same  treatment  in  every  way,  they  will  still  be  very 
different.  It  is  a  natural  quality  of  sweet  corn,  as  compared 
with  field  corn,  to  produce  a  smaller  stalk  and  to  have  more 
sugar  in  the  grain;  and  these  differences  will  appear  even 
when  the  environments  are  the  same.  We  have,  therefore, 


224  Science  of  Plant  Life 


U.  S.  Dept.  of  Agriculture 

FIG.  140.  Tobacco  plants  of  the  same  variety  grown  from  large,  medium,  and  small 
seeds,  showing  the  relation  between  the  size  of  the  seed  and  the  size  and  vigor  of  the 
seedling.  Is  the  difference  in  size  in  the  plants  due  to  environment  or  to  differences 
in  the  plants  themselves? 

two  kinds  of  variations  in  plants :  variations  due  to  environ- 
ment and  variations  due  to  differences  in  the  plants  themselves. 

All  variations  are  of  interest  to  the  plant  breeder ;  but  since 
he  is  trying  to  produce  new  plants  that  may  be  perpetuated, 
he  is  most  interested  in  those  variations  which  are  inherited, 
-that  is,  variations  which  may  be  carried  over  from  one 
generation  to  another.  There  is  little  or  no  evidence  that 
variations  produced  by  environment  may  be  inherited;  a 
variety  of  corn  grown  for  many  generations  on  rich  soil  would 
develop  no  larger  plants  on  poor  soil  than  would  corn  of  the 
same  variety  that  had  always  been  grown  on  poor  soil. 

Mutation.  '  Sometimes,  among  many  thousands  of  individ- 
uals, a  single  plant  appears  which  is  markedly  different  from 
all  the  others.  For  example,  a  few  years  ago  a  sunflower  was 
discovered  that  had  some  red  pigment  near  the  base  of  the 
otherwise  yellow  corollas.  Among  the  millions  of  sunflowers 


Reproduction  in  Relation  to  Agriculture        225 

that  have  been  seen,  this  was  the  first  one  in  which  a  red 
color  was  noticed.  In  some  unknown  way  there  was  produced 
in  this  plant  a  red  pigment  not  formed  in  other  sunflowers. 
From  the  seeds  of  this  plant  there  were  developed  other  plants 
having  red  pigment  in  their  flowers.  Evidently  the  new 
character  is  inherited  and  these  sunflowers  have  a  chemical 
constitution  which  enables  red  pigment  as  well  as  yellow  to 
be  formed.  The  sudden  appearance  of  the  sunflower  with 
the  red  pigment  is  an  example  of  mutation.  Individuals  that 
first  show  new  characters  are  called  mutants  (Latin :  mutarej 
to  change). 

What  the  plant  breeders  have  long  known  as  "  sports  "  are 
the  rare  mutants  in  which  notable  changes  have  occurred. 
They  show  new  characters,  and  these  characters  are  in- 
herited. Consequently,  their  discovery  is  of  the  greatest 
importance. 

The  Concord  grape  was  a  mutant  selected  from  among  many 
seedlings,  the  others  of  which  showed  only  the  usual  variations. 
The  many  modern  varieties  of  tomato  have  been  developed 


FIG.  141.     Varieties  derived  from  the  wild  cabbage  (F),  a  native  plant  of  Europe. 
A  is  kohl-rabi,  B  cabbage,  C  cauliflower,  D  kale,  and  E  Brussels  sprouts. 


226  Science  of  Plant  Life 

from  mutations  that  occurred  among  the  currant  tomatoes 
or  love  apples  grown  for  ornament  in  our  great-grandmothers' 
gardens.  The  original  fruits  resembled  large  red  currants. 
Today  single  tomato  berries  may  weigh  a  pound.  In  color 
they  may  be  red,  yellow,  or  pink,  and  in  shape  they  may  be 
spherical,  plum-shaped,  pear-shaped,  or  flattened.  They 
exhibit  at  least  three  types  of  leaves  and  two  types  of  stems. 
The  characteristics  due  to  mutation  are  inherited,  no  matter 
what  the  soil  and  climatic  conditions  may  be. 

Mutations  occur  not  only  among  plants  grown  from 
seed,  but  also  among  plants,  or  plant  parts,  developed  from 
buds.  These  are  called  bud  mutations,  or  bud  sports.  On 
fruit  trees,  one  branch  will  occasionally  produce  fruit  that 
is  of  different  quality  from  the  fruit  produced  on  other 
branches.  If  the  quality  of  the  fruit  is  superior,  these 
branches  may  be  used  in  budding  and  grafting  to  preserve 
the  new  variety.  Bud  sports  are  comparatively  rare,  but  it  is 
estimated  that  at  least  several  hundred  horticultural  varieties 
have  originated  from  them.  In  this  country  the  improved 
varieties  of  navel  oranges  have  been  secured  entirely  by  this 
method.  The  Boston  fern  and  its  forty  or  more  varieties 
originated  in  bud  mutations  from  a  wild  tropical  fern.  In  the 
potato  and  some  other  plants  that  are  usually  propagated 
vegetatively,  bud  variations  are  known  to  occur;  but  they 
are  so  rare  and  so  difficult  to  discover  in  plants  of  this  kind 
that  they  have  not  been  of  much  practical  value. 

Hybridization.  The  crossing  of  two  species  or  varieties  of 
plants  is  known  as  hybridization.  It  is  brought  about  by 
transferring  the  pollen  from  one  to  the  stigma  of  the  other. 
The  plants  grown  from  seed  produced  in  this  way  are  called 
hybrids.  Hybrids  may  resemble  one  of  the  parents,  or  they 


Reproduction  in  Relation  to  Agriculture        227 


U.  S.  Dept.  of  Agriculture 

FIG.  142.     Vegetable  trial  grounds  of  the  Office  of  Seed  and  Plant  Introduction, 
United  States  Department  of  Agriculture,  Washington,  D.  C. 


•'••"j&tefe, 


•      .    -.:•:• 

••!.  I 

*«•• 


U.  S.  Dept.  of  Agriculture 

FIG.  143.     Bulb  trial  grounds  of  the  Office  of  Seed  and  Plant  Introduction,  United 
States  Department  of  Agriculture,  Washington,  D.  C. 


228  Science  of  Plant  Life 

may  show  some  characters  of  each.  In  the  second  generation 
derived  from  crosses  some  show  a  wide  range  of  variation,  with 
all  possible  combinations  of  the  characteristics  of  the  parent 
plants.  Successful  hybridization,  therefore,  frequently  in- 
creases the  number  of  variations  available  for  selection  by 
the  plant  breeder. 

In  many  plants  hybridizing  has  a  physiological  effect 
which  is  of  importance,  in  that  it  increases  the  vigor  of  the 
offspring.  In  sunflowers,  for  example,  the  hybrids  secured 
by  crossing  the  American  sunflower  and  the  Russian  sun- 
flower, neither  of  which  was  over  10  feet  in  height,  grew 
under  the  same  conditions  to  a  height  of  15  feet. 

Selection.  Variations,  mutations,  and  the  results  of  hy- 
bridization furnish  the  material  from  which  valuable  new 
varieties  of  animals  and  plants  may  be  selected.  The  plant 
breeder  selects  from  among  the  hundreds  or  thousands  of 
plants  grown  in  trial  grounds  those  individuals  that  most 
nearly  approach  the  form  or  quality  desired.  The  seeds  of 
these  plants  are  kept  separate  and  are  planted  the  following 
season.  This  process  of  selection  may  be  repeated  year  after 
year  until  a  large  part  of  the  progeny  or  all  of  them  show  the 
quality  aimed  at  in  the  selection.  Some  variations  can  be 
maintained  only  by  continual  selection  of  the  best  individuals. 
Others  may  soon  become  stable  in  character,  and  the  plants 
are  then  said  by  plant  breeders  to  breed  true.  In  fruit  trees, 
desirable  varieties  may  be  maintained  by  grafting  and 
budding ;  and  in  other  plants  that  can  be  propagated  vegeta- 
tively,  varieties  may  be  perpetuated  by  cuttings,  bulbs, 
tubers,  and  corms. 

Weeds.  Another  way  in  which  agriculture  is  affected  by 
reproduction  in  plants  is  through  the  multiplication  of  un- 


Reproduction  in  Relation  to  Agriculture        229 


FIG.  144.  Fiber  from  new  varieties  of  long-fibered  cotton  at  the  right,  obtained 
by  hybridizing  and  selecting  progeny  from  the  two  forms  producing  the  shorter 
fibers  at  the  left.  The  hybrid  offspring  excel  both  parents  in  the  length  of 

fiber  produced. 

» 

desirable  plants.  Weeds  as  a  class  are  plants  in  which  re- 
production has  reached  the  highest  degree  of  efficiency.  The 
sequoia  may  stand  for  the  culmination  of  vegetative  efficiency, 
the  dandelion  for  efficiency  in  reproduction  and  dispersal. 
The  dandelion  produces  good  seed  without  pollination ;  if  the 
stem  is  cut,  the  plant  develops  numerous  new  sprouts;  if 
the  root  is  cut  into  small  pieces,  each  piece  may  sprout 
from  either  end  or  from  both  ends  at  the  same  time.  The 
dandelion  can  thrive  in  a  swamp,  and  it  can  withstand  the 
droughts  of  a  sand  plain.  The  sequoia  still  occupies  the 
comparatively  small  area  in  which  it  existed  several  thousand 
years  ago.  It  reproduces  very  slowly,  and  it  is  restricted  to 
a  single  habitat.  The  dandelion  has  in  recent  times  spread 
to  all  parts  of  the  world,  and  it  occurs  in  most  habitats, 
from  the  seashore  to  the  alpine  summits  of  mountains.  In 
other  words,  it  becomes  adjusted  to  many  environments. 


230  Science  of  Plant  Life 

In  the  ordinary  sense  the  term  "  weed  "  applies  to  any  un- 
desirable plant  that  reproduces  abundantly  and  that  adjusts 
itself  to  diverse  habitats.  In  its  broadest  sense  the  term  is 
applied  to  any  plant  growing  out  of  place.  Rye  may  become 
a  weed  in  wheatfields.  Red  clover  is  very  desirable  in  a  field 
on  the  farm,  but  it  becomes  a  weed  when  it  springs  up  in  a 
lawn.  The  most  pernicious  weeds,  like  the  dandelion,  cockle 
bur,  Canada  thistle,  bindweed,  and  sand  bur,  are  not  desir- 
able plants  anywhere. 

Weeds  interfere  with  crop  production.  Weeds  are  un- 
desirable and  harmful,  (i)  because  they  use  soil  water  needed 
by  crop  plants,  (2)  because  they  interfere  with  the  growth  of 
crops  by  shading  them  and  by  occupying  the  land,  (3)  be- 
cause some  weeds  promote  the  spread  of  diseases  and  injurious 
insects,  and  (4)  because  some  of  the  species  found  in  pastures 
and  grazing  lands  are  poisonous,  or -harmful  to  cattle,  or 
spoil  the  quality  of  milk,  butter,  and  cheese  by  their  persistent 
and  unpleasant  flavors.  It  is  estimated  that  the  average 
American  farmer  loses  annually  one  dollar  on  every  acre  of 
his  land  through  the  damage  done  by  weeds. 

Weeds  spread  rapidly.  How  rapidly  a  weed  may  spread 
is  illustrated  by  the  history  of  the  Russian  thistle.  It  was 
introduced  into  South  Dakota  in  imported  flaxseed,  in  1874, 
and  it  had  become  a  common  weed  by  1888.  In  1894  it  had 
spread  as  far  as  Chicago.  In  1900  it  was  reported  in  all  the 
states  and  provinces  east  of  the  Rockies,  from  the  Gulf  of 
Mexico  and  Atlantic  Ocean  to  Saskatchewan. 

The  control  of  weeds.  In  crops  that  are  cultivated  at 
intervals,  most  weeds  can  be  controlled  by  turning  under  the 
seedlings  as  fast  as  they  develop.  In  fields  of  wheat  and 
other  small  grains  the  problem  of  weed  control  is  more  dif- 


Reproduction  in  Relation  to  Agriculture        231 

licult.  By  changing  the  crop  from  year  to  year  so  as  to  fol- 
low or  precede  the  crop  of  small  grain  with  a  crop  like  corn, 
that  is  cultivated  at  intervals  during  its  development,  the 
weeds  may  to  some  extent  be  held  in  check.  Grazing  animals, 
particularly  sheep,  may  aid  greatly  in  removing  weeds  from 
pastures. 

In  grainfields,  pasture  lands,  and  meadows  some  success  in 
weed  control  has  recently  been  attained  by  the  use  of  solu- 
tions of  poisons,  particularly  copper  sulfate  and  iron  sulfate. 
The  fields  are  sprayed  with  the  poisonous  solution  while  the 
plants  are  young.  Grasses  are  not  greatly  injured  by  the 
spray  and  recover  quickly,  while  weeds  like  the  dandelion, 
wild  mustard,  and  corn  cockle  are  killed. 

In  order  to  control  the  weeds  in  sugar-cane  fields,  some  fields 
are  now  covered  with  paper.  The  strong,  spear  like  shoots  of 
the  sugar  cane  break  through  the  paper,  but  the  weed  seed- 
lings do  not.  The  paper  is  made  from  the  pulp  left  after  the 
sap  has  been  pressed  out  from  the  cane.  Formerly  this  pulp 
was  wasted,  but  now  it  has  been  put  to  good  use  in  the  form  of 
field  paper. 

Weeds  should  not  be  allowed  to  ripen  seeds.  The  state 
laws  which  require  that  weeds  be  cut  along  the  roads  aim  not 
so  much  at  the  removal  of  the  unsightly  plants  from  the 
roadsides  as  at  the  protection  of  farmers  from  the  seeds. 

Seed  that  is  to  be  planted  on'  any  farm  should  be  examined 
for  weed  seeds.  If  they  are  present,  the  seed  should  be  either 
cleaned  or  rejected.  Many  a  farmer  plants  his  weeds  with  his 
grain. 


232  Science  of  Plant  Life 


PROBLEMS 

1.  How  much  of  the  potato  must  be  planted  with  each  eye  in  order  to  get  normal 
growth? 

2.  In  what  plants  would  it  be  profitable  to  select  varieties  for  increased  size  of 
the  flower  cluster? 

3.  Why  should  field  corn  and  sweet  corn  be  widely  separated  when  grown  on  the 
same  farm? 

4.  What  are  the  most  important  things  to  know  about  seed,  when  buying  it 
for  the  farm? 

5.  In  a  row  of  radishes  a  few  will  develop  bulbous  roots  before  the  others. 
Would  seed  from  these  produce  an  earlier  maturing  crop  than  seed  taken 
at  random  from  the  row? 

6.  In  what  plants  would  it  be  profitable  to  select  for  increased  height  of  stem? 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Eighteen 

1.  Make  a  field  study  of  Protococcus  to  determine  (i)  where  it 
grows,  (2)  its  relation  to  light,  moisture,  gravity,  and  roughness 
of  the  substratum,  and  (3)  how  it  is  disseminated. 

2.  Make  a  field  study  of  pond  scums  in  a  pond  or  small  stream. 
In  a  jar,  collect  material  for  laboratory  study ;  cover  the  bottom 
of  the  jar  with  mud  from  the  bottom  of  a  pond  or  stream.     The 
amount  of  algae  should  be  very  small  compared  with  the  volume  of 
water.     If  these  algae  are  collected  in  late  autumn,  they  may  be 
grown  in  winter  at  a  north  window. 

3.  The  forms  of  the  cells  and  the  methods  of  reproduction  may 
be  studied  from  fresh  or  preserved  material.     Particular  attention 
should  be  given  to  the  differences  between  vegetative  multiplica- 
tion, sexual  reproduction,  and  asexual  reproduction. 

4.  Dried   specimens   of   Fucus,   Laminaria,    Sargassum,  Poly- 
siphonia,  and  Dasya  will  give  an  idea  of  the  Brown  and  Red  Algae. 

5.  In  early  spring  place  some  algae  in  an  aquarium  with  snails 
and  young  tadpoles.     Note  the  rate  at  which  the  algae  disappear. 
If  you  can  find  frog  or  toad  eggs,  the  experiment  may  be  started 
with  them.     The  water  should  be  changed  occasionally. 


233 


CHAPTER   EIGHTEEN 

THE  ALGJE 

THE  plants  that  we  have  discussed  in  the  preceding  chapters 
are  all  seed  plants,  —  plants  with  well-developed  roots,  stems, 
leaves,  flowers,  and  seeds.  In  these  plants  the  physiological 
processes  —  photosynthesis,  digestion,  absorption,  conduction, 
accumulation,  and  reproduction  —  are  carried  on  in  tissues 
and  organs  that  are  specialized  to  varying  degrees,  and  the 
plant  body  is  a  complex  structure  in  which  all  the  organs  are 
mutually  dependent.  The  seed  plants  make  up  by  far  the 
largest  and  most  conspicuous  part  of  the  earth's  vegetation,  and 
it  is  with  them  that  the  word  "  plant  "  is  ordinarily  associated. 

There  are  thousands  of  other  plants,  however,  that  are 
far  simpler  in  structure.  They  usually  pass  unnoticed  among 
the  larger  plants  that  crowd  the  landscape;  but  this  does 
not  imply  that  they  are  unimportant  in  the  world  of  plants 
and  animals.  Indeed,  quite  the  reverse  is  true.  They  play 
a  very  definite  role  in  nature  :  they  modify  the  earth's  surface, 
supply  food,  and  produce  other  effects  which  are  of  great  con- 
sequence to  the  seed  plants,  to  animals,  and  to  man.  In  this 
and  the  succeeding  chapters  we  shall  make  a  brief  study  of 
these  simple  plants  in  order  that  our  conception  of  the  plant 
kingdom  as  a  whole  may  be  more  complete,  and  also  that  we 
may  gain  some  appreciation  of  the  relations  of  man,  of  the 
animals,  and  of  the  complex  plants  to  the  lower  forms  of 
plant  life.  The  first  group  that  we  shall  study  are  the  alga, 
small  green  plants  that  are  very  common  in  ponds,  brooks, 
and  pools,  but  which  are  found  in  other  habitats  also.  The 
algae  occur  in  all  parts  of  the  earth. 

The  simple  structure  of  the  algae.  The  algae  are  simple 
plants  that  consist  of  single  cells,  groups  of  cells,  or  masses  of 

234 


The  Algae  235 

cells.  In  the  lowest  algae  the  cells  are  quite  independent  of 
one  another,  and  in  most  of  them  dependence  among  the 
cells  of  the  plant  body  is  very  slight.  That  is,  in  most  forms, 
organs  corresponding  to  the  roots,  stems,  leaves,  conducting 
systems,  and  reproductive  parts  of  higher  plants  are  not 
found,  and  all  the  cells  in  the  plant  body  are  much  alike. 
Under  these  conditions  each  cell  must  carry  on  for  itself  the 
processes  of  absorption,  photosynthesis,  digestion,  assimi- 
lation, and  respiration.  So,  when  we  speak  of  the  algae  as 
simple  plants,  we  mean  only  that  they  are  simple  in  structure  ; 
in  their  physiological  processes  they  may  be  very  complex. 
They  all  contain  chlorophyll  and  are  therefore  capable  of 
manufacturing  food.  Some  of  them,  however,  grow  better 
in  water  that  contains  organic  matter,  and  these  forms  doubt- 
less get  a  part  of  their  food  directly  from  the  organic  sub- 
stances in  solution  in  the  water.  A  good  type  with  which  to 
begin  a  study  of  algae  is  Protococcus,  a  very  common  and  ex- 
tremely simple  form. 

Protococcus.  On  the  partly  shaded,  moist  sides  of  trees, 
rocks,  buildings,  and  fences  everywhere,  there  occur  patches 
that  look  as  if  they  had  been  stained  green.  If  a  little  of 
this  stain  is  scraped  off  and  examined  under  a  microscope,  it 
is  seen  to  be  made  up  of  little  rounded  green  cells.  Each  cell 
has  a  cell  wall,  cytoplasm,  and  nucleus.  In  the  cytoplasm  is 
a  large  green  plastid  which  almost  fills  the  cell. 

When  the  cells  are  examined,  certain  of  them  will  be  found 
to  be  elongated ;  some  of  these  may  be  dividing  into  two. 
Sometimes  there  are  two  or  more  cells  still  clinging  together, 
showing  clearly  that  they  have  just  been  formed  by  division. 
These  groups  separate  readily  when  the  cover  glass  is  tapped, 
and  each  single  cell  may  go  on  living  quite  independently  of 


236 


Science  of  Plant  Life 


FIG.  145.  Freshwater  algae.  The  upright  filaments  are,  from  left  to  right: 
(Edogonium,  producing  swimming  spores,  eggs,  and  sperms;  Microspora,  form- 
ing resting  spores  and  swimming  spofes;  and  Ulothrix,  forming  swimming 
spores  and  gametes.  The  horizontal  filaments  are  Spirogyra  (left)  and 
Vaucheria  (right). 


The  Algae  237 

the  others.  The  plant,  therefore,  consists  of  a  single  cell 
which  carries  on  all  the  essential  processes  of  life  and  is  able 
to  reproduce  itself.  Moreover,  it  is  a  highly  successful  plant, 
for  Protococcus  occurs  in  all  parts  of  the  world,  from  the 
tropics  to  the  polar  regions,  in  habitats  of  many  different 
kinds. 

The  pond  scums.  If  examined  in  the  spring  or  fall,  al- 
most every  pond  and  little  stream  will  be  found  to  contain 
many  kinds  of  algae.  Some  of  these  are  merely  masses  of 
rounded  cells  like  the  cells  of  Protococcus.  Others  have  the 
cells  arranged  in  rows,  forming  simple  filaments.  In  still 
others  the  filaments  are  highly  branched  and  the  plant  body 
may  be  several  feet  in  length.  Some  of  the  forms  are  im- 
bedded in  a  gelatinous  matrix.  All  these  various  kinds  of 
algae  taken  together  are  called  the  "  pond  scums."  They  are 
the  algae  that  a  person  who  has  not  studied  botany  is  most 
likely  to  know. 

Many  of  the  pond  scums  are  at  first  attached  to  under- 
water objects,  but  during  warm  weather  they  break  loose  and 
come  to  the  surface.  All  cells  carrying  on  photosynthesis 
give  off  oxygen,  and  the  bubbles  of  oxygen  that  come  from 
the  filaments  cling  to  them  and  help  to  buoy  them  up.  Fur- 
thermore, bubbles  of  air  which  are  given  off  from  the  water 
when  it  becomes  warm  collect  under  the  masses  of  algae  and 
cause  them  to  come  to  the  top  of  the  water,  where  they  form 
a  green  or  yellowish-green  surface  layer.  The  pond  scums 
are  generally  considered  unsightly,  and  not  a  few  persons 
think  them  poisonous.  In  reality,  they  are  quite  as  harm- 
less as  lettuce.  The  danger  in  drinking  from  ponds  lies  not 
in  the  green  scums,  but  in  the  presence  of  certain  disease- 
producing  bacteria  that  may  have  been  carried  into  the  ponds 


238  Science  of  Plant  Life 

by  surface  water.  Several  thousand  different  species  of  algae 
are  concerned  in  the  formation  of  pond  scums.  Microspora 
may  be  studied  as  an  example  of  the  more  simple  filamentous 
forms. 

Microspora.  The  Microspora  plant  is  a  filament  made  up 
of  cylindrical  or  barrel-shaped,  cells  placed  end  to  end.  Each 
cell  carries  on  all  its  own  food-  and  energy-producing  pro- 
cesses. During  early  spring,  as  food  is  manufactured,  the 
cells  enlarge  and  divide.  The  division  is  always  in  the  same 
direction,  however,  and  the  cells  remain  attached  to  each 
other,  so  that  the  growth  and  division  of  the  cells  causes  the 
filament  to  increase  in  length.  This  long,  slender  line  of 
cells  is  easily  broken,  and  the  plant  may  be  multiplied  by  the 
breaking  of  the  filaments  into  parts. 

Spores  in  Microspora.  Microspora  produces  swimming 
spores  and  resting  spores.  These  are  special  cells  that  re- 
produce the  plant.  A  swimming  spore  is  formed  by  the 
contents  of  a  cell  in  the  filament  contracting  into  an  ovoid 
body.  At  one  end  of  this  body  two  cilia,  which  are  small, 
hairlike  propellers,  are  developed.  The  wall  of  the  original 
cell  then  breaks  and  the  swimming  spore  is  set  free.  After 
swimming  about  in  the  water  for  a  short  time,  it  becomes 
attached  to  some  object  under  water,  loses  its  cilia,  and 
grows  into  a  cylindrical  vegetative  cell.  This  cell  then  con- 
tinues to  grow  and  divide  until  a  new  filament  is  formed.  The 
advantages  of  swimming  spores  are  that  they  multiply  the 
plant,  and  by  their  ability  to  swim  they  enable  the  plant  to 
spread  to  new  locations  that  it  might  not  reach  without  these 
motile  cells. 

The  resting  spores  are  usually  formed  in  the  spring  after 
the  active  period  of  vegetative  growth  has  passed.  At  this 


The  Algae  239 

season  the  cells  in  the  filament  stop  dividing  and  food  accumu- 
lates in  the  form  of  starch  and  protein  granules.  The  proto- 
plasm in  each  cell  then  contracts  into  a  spherical  form  and 
secretes  a  heavy  cell  wall  about  itself  inside  the  original  cell 
wall.  In  this  way  the  cells  of  a  filament  form  a  row  of  ovoid 
or  spherical  heavy- walled  resting  spores.  Usually  the  walls 
of  these  spores  become  yellow  or  brownish.  The  resting  spore 
remains  dormant  until  the  late  fall  or  early  spring.  Then, 
like  a  seed,  it  germinates,  or  begins  to  grow.  The  wall  that 
incloses  the  spore  breaks  and  the  protoplasm  pushes  out  and 
forms  a  cylindrical  vegetative  cell  which  continues  to  grow 
and  divide  until  a  new  filament  is  formed. 

Microspora,  then,  in  addition  to  the  vegetative  multipli- 
cation of  the  cells  shown  by  Protococcus,  has  specialized 
swimming  spores  that  multiply  and  spread  the  plant,  and 
resting  spores  that  undergo  a  dormant  period,  after  which, 
when  favorable  conditions  for  growth  appear,  they  produce 
a  new  plant.  Its  life  cycle  and  that  of  other  similar  algae 
includes  (i)  an  active  chlorophyll- working  period,  during 
which  the  plant  grows  and  enlarges  its  body  and  accumulates 
food ;  (2)  a  reproductive  phase,  which  closes  with  the  pro- 
duction of  resting  spores ;  and  (3)  a  period  of  dormancy,  dur- 
ing which  only  the  resting  spores  are  alive.  The  length  of 
the  dormant  period  for  a  particular  alga  is  practically  the  same, 
whether  it  lives  in  a  permanent  pond  or  in  a  pool  that  dries 
up  in  summer. 

The  living  conditions  of  the  pond  algae.  Curiously,  the 
ponds  in  which  the  algae  are  most  abundant  are  the  ones  that 
dry  up  in  the  summer.  Yet  these  plants  are  among  the  most 
delicate  of  all  living  things ;  you  may  readily  discover,  by 
putting  the  green  masses  in  a  dish  and  letting  the  water 


240  Science  of  Plant  Life 

• 

evaporate,  that  the  vegetative  plants  are  killed  by  drying. 
How,  then,  do  they  withstand  the  dry  period  ? 

It  is  resting  spores  that  enable  the  plant  to  be  carried  over 
the  dry  season.  These  spores  are  produced  in  large  numbers 
and,  being  heavier  than  water,  sink  and  become  a  part  of  the 
mud  bottom.  When  the  pond  dries  up,  the  spores  are  in- 
cased in  the  dried  mud.  At  intervals  they  are  watered  by 
rains,  but  they  do  not  germinate  for  weeks  or  months  after 
their  formation.  No  doubt  a  large  part  of  the  spores  that  were 
formed  perish  during  this  period,  but  enough  survive  a  drought 
to  start  the  plant  the  following  season. 

Ulothrix.  Another  green  alga  occurring  on  the  margins  of 
lakes,  in  running  streams,  and  in  clear  springs  is  Ulothrix. 
It  has  a  filamentous  body  similar  in  many  respects  to  Micro- 
spora,  and  like  that  form  it  is  attached  to  rocks  and  other 
objects.  Its  methods  of  reproduction,  however,  are  more 
numerous  and  more  complex  than  those  of  Microspora,  and 
they  will  serve  to  exemplify  the  reproductive  processes  of 
many  other  forms  of  algae.  When  the  filaments  are  mature, 
the  protoplasm  within  some  of  the  cells  divides  into  two, 
four,  or  eight  parts,  each  of  which  contains  nucleus,  cytoplasm, 
chloroplast,  and  vacuole.  Each  of  these  parts  becomes  oval 
in  shape  and  develops  into  a  swimming  spore  with  four  cilia. 
An  opening  appears  at  one  side  of  the  original  cell  wall,  and 
a  few  minutes  later  the  swimming  spores  pass  out  from  the 
cell  cavity  and  swim  away.  Sometimes  all  the  cells  in  a 
filament  produce  swimming  spores  at  about  the  same  time, 
and  hundreds  of  these  small  green  bodies  may  be  found 
moving  about  in  the  water.  At  the  end  of  from  15  to  30 
minutes  the  swimming  spores  settle  down  on  some  object 
and  become  attached.  By  the  end  of  a  day  the  cell  formed 


The  Algae  241 

from  each  spore  has  divided  and  produced  the  first  two  cells 
of  a  new  filament. 

The  protoplasm  of  other  cells  of  the  same  or  other  filaments 
continues  to  divide  until  16,  32,  64,  or  more  bodies  have  been 
formed.  These  are  called  gametes.  They  are  similar  to  the 
swimming  spores  but  much  smaller,  and  each  possesses  two 
cilia  for  swimming.  Like  a  swimming  spore,  each  of  them 
leaves  the  old  cell  through  an  opening  in  the  wall.  The 
gametes  swim  about  for  some  minutes  and  then  unite  in  pairs. 
They  are  attached  at  first  only  by  the  ciliated  ends,  but  later 
the  two  gametes  fuse.  The  body  thus  formed  may  grow 
directly  into  a  new  filament,  or  it  may  produce  swimming 
spores  from  each  of  which  a  new  filament  is  formed. 

Asexual  and  sexual  spores.  Spores  of  Microspora  and  the 
swimming  spores  of  Ulothrix  are  formed  directly  from  vegeta- 
tive cells  or  by  the  division  of  the  contents  of  these  cells.  The 
spore  which  is  formed  by  the  -two  gametes  of  Ulothrix  is  pro- 
duced by  the  union  of  two  cells.  Since  cell  division  takes 
place  in  all  parts  of  plants  while  cell  union  occurs  only  in  the 
sexual  process,  it  is  plain  that  we  have  in  Ulothrix  a  simple 
type  of  sexual  reproduction.  The  spore  that  is  formed  by 
the  union  of  the  gametes  is,  therefore,  a  sexual  spore.  The 
swimming  spores  and  the  resting  spores  are  formed  without 
the  union  of  cells,  and  these  are  called  asexual  spores.  The 
gametes  of  Ulothrix  look  alike,  but  physiologically  they  are 
probably  different.  One  of  them  corresponds  to  the  male 
reproductive  cell  of  the  higher  plants,  and  the  other  to  the 
female  reproductive  cell  (page  205). 

(Edogonium.  (Edogonium  is  another  filamentous  alga 
that  flourishes  in  ponds  and  streams.  In  early  life  the  fila- 
ments are  attached,  but  large  masses  of  them  will  often  be 


242  Science  of  Plant  Life 

iound  free  in  ponds  and  stagnant  pools.  From  the  cylindrical 
vegetative  cells,  large  swimming  spores  are  formed.  Gametes 
also  are  produced.  These  are  of  two  distinct  forms,  male 
and  female.  Plants  belonging  to  the  (Edogonium  group  may 
be  used  to  exemplify  reproduction  in  many  other  algae,  whose 
gametes  are  essentially  like  those  of  more  complex  plants. 

At  the  time  of  production  of  the  gametes,  some  of  the  cells 
in  the  filament  enlarge,  become  rounded,  and  accumulate 
starch  and  other  food  material ;  also,  a  small  opening  is  formed 
in  the  cell  wall.  The  content  of  this  cell  is  the  female  gamete 
or  egg,  which  like  other  eggs  has  in  it  a  store  of  food. 

Other  cells  of  the  filament  are  cut  up  into  very  short  cells 
by  the  formation  of  transverse  walls.  In  each  of  these  short 
cells  there  are  formed  two  small  gametes,  which  escape  from 
the  filament  and  swim  out  into  the  water.  These  are  the 
male  gametes,  or  sperms.  Fertilization  takes  place  when  one 
of  the  sperms  enters  through  the  opening  in  the  cell  wall  that 
surrounds  an  egg  and  unites  with  the  egg.  The  egg  and  the 
sperm  may  be  of  the  same  filament  or  of  different  filaments. 
The  product  is  a  sexual  spore,  usually  called  an  oospore  (egg 
spore).  After  a  dormant  period  this  produces  swimming 
spores  that  start  new  filaments. 

In  (Edogonium,  therefore,  the  sex  cells  are  of  two  kinds 
quite  distinct  in  structure  and  function.  The  egg  is  a  large, 
stationary  cell  filled  with  food.  The  sperm  is  a  small,  swim- 
ming cell  that  moves  to  the  egg  and  accomplishes  fertilization 
by  uniting  with  it.  The  product  is  an  oospore  which  may 
start  a  new  generation  of  plants  like  that  which  produced  it. 
This  condition  is  essentially  like  that  in  the  seed  plant,  where 
the  egg  is  produced  inside  a  complex  structure  called  the 
ovule  and  the  sperm  reaches  it  through  a  pollen  tube  (page 


The  Algae  243 

205).  When  the  two  unite  in  fertilization,  the  product  in  a 
seed  plant  also  is  an  oospore,  or  fertilized  egg,  which  im- 
mediately divides  and  forms  the  embryo  within  the  seed. 

Reproduction  among  the  algae.  The  methods  of  repro- 
duction among  the  algae  that  we  have  studied  are  representa- 
tive of  those  found  in  the  entire  group.  The  three  general 
types  are : 

(1)  Vegetative  multiplication.     By  means  of  cell   division 
cell  masses,  filaments,  or  highly  branched  plant  bodies  are 
produced.     If  the  individual  cells  separate  from  each  other 
after  division,  as  in  Protococcus,  many  new  individual  plants 
are  produced ;    and  when  filaments  and  branched  forms  are 
broken,  as  in  Microspora,  a  new  individual  plant  is  produced 
by  each  part. 

(2)  Reproduction  by  asexual  spores.     Vegetative  cells  form 
thick-walled  resting  spores  which  carry  the  plant  over  to  the 
next  season.     Another  kind  of  asexual  spore  is  the  swimming 
spore,  by  means  of  which  the  plant  secures  immediate  repro- 
duction and  spreads  to  other  parts  of  the  pond  or  stream. 
These  spores  are  formed  directly  from  vegetative  cells  or  by 
the  division  of  vegetative  cells.     There  is  no  union  of  cells  as 
there  is  when  sexual  spores  are  formed. 

(3)  Sexual  reproduction.     A   sexual   spore,  or   oospore,  is 
formed  by  the  union  of  two  gametes.     The  gametes  may  be 
similar  in  size  and  appearance,  as  in  Ulothrix,  or  they  may  be 
unlike,  as  in  (Edogonium,  where  one  gamete  accumulates  a 
large  food  supply  and  the  other  is  small  and  motile.     In  either 
case,  the  one  gamete  corresponds  to  the  sperm  and  the  othd 
to  the  egg  that  is  found  in  higher  plants.     The  union  is  the 
process  of  fertilization.     The  oospore  may  germinate  imme- 
diately, but  more  often  it  remains  dormant  for  a  period  of 


244 


Science  of  Plant  Life 


FIG.  146.     Food  relations  of  aquatic  life. 

weeks  or  months.     Essentially  this  same  process  occurs  in 
the  sexual  reproduction  of  all  plants  and  animals. 

Other  algae  found  among  pond  scums.  Great  numbers  of 
different  kinds  of  algae  are  found  growing  in  fresh- water  ponds 
and  pools,  and  when  these  are  examined  under  the  microscope 
they  are  seen  to  be  composed  of  the  most  beautiful  and  the 
most  varied  cells  found  in  the  whole  plant  kingdom.  Some 
have  intricate  star-shaped  and  latticelike  chloroplasts ;  in 
other  forms  the  chloroplasts  wind  about  within  the  cells  like 
spiral  green  ribbons.  Some  forms  have  no  cell  walls  in  the 
filament,  but  all  the  living  matter  of  the  plant,  with  hun- 
dreds of  nuclei  in  it,  lies  together  in  one  mass.  A  great  group 
of  one-celled  forms  (Diatoms)  have  siliceous  cell  walls  that 
are  like  delicate  cases  of  glass  and  that  are  sculptured  and 
marked  with  lines  in  a  highly  complex  way.  Some  of  these 
forms  are  stationary,  while  others  move  slowly  through  the 
water  by  invisible  means.  Still  other  forms  swim  about 


The  Algae  245 

rapidly  by  means  of  little  hair  like  appendages,  seeming  more 
like  animals  than  plants. 

The  pond  scums  include  many  blue-green  algae,  which  are 
much  simpler  in  structure  than  the  green  plants  that  we  have 
studied.  These  contain,  in  addition  to  chlorophyll,  a  blue- 
green  pigment  which  gives  them  the%  color  from  which  the 
group  takes  its  name.  The  cells  are  generally  smaller  than 
those  in  the  green  algae,  and  distinct  nucleii  and  chloroplasts 
are  absent.  These  small  blue-green  forms  are  common  in  all 
kinds  of  water  habitats  and  also  in  the  upper  layers  of  the 
soil.  Sometimes  during  wet  weather  they  become  sufficiently 
abundant  in  fields  to  give  a  blue-green  color  to  the  soil. 

The  importance  of  the  pond  scums.  Both  green  and  blue- 
green  algae  are  generally  considered  a  nuisance  in  ponds  and 
streams,  and  they  are  commonly  thought  to  have  no  economic 
importance ;  but  the  fact  is  that  these  despised  pond  scums 
are  the  primary  food  supply  of  all  the  water  animals.  They 
bear  the  same  relation  to  aquatic  animal  life  that  the  her- 
baceous plants  bear  to  animal  life  on  the  land.  Nearly  all  the 
water  animals,  from  minute  insects  and  crustaceans  to  the 
largest  fishes,  ultimately  depend  upon  them  for  their  supply 
of  food.  For,  like  the  land  plants,  these  small  water  plants 
manufacture  food,  and  the  animals  that  live  in  the  water 
must  feed  either  on  them  or  on  other  animals  that  get  their 
living  from  the  plants.  Without  the  pond  scums  the  fish 
would  soon  disappear  from  our  waters,  because  their  food 
supply  would  be  cut  off.  A  decrease  in  the  number  of  fish  in 
a  lake  frequently  follows  the  draining  of  its  swampy  margins, 
for  the  algae  thrive  best  in  shallow  water,  and  it  is  from  the 
algae  that  the  small  animals  on  which  the  fish  feed  secure 
their  food.  The  time  is  not  far  distant  when  fish  will  be 


246  Science  of  Plant  Life 

more  highly  prized  as  food  than  they  are  now.  When  that 
time  comes,  the  cultivation  of  algae  will  probably  be  under- 
taken as  a  first  step  toward  greater  fish  production. 

But  while  the  pond  scums  are  a  source  of  food  for  water 
animals,  they  are  also  a  source  of  annoyance  in  reservoirs  in 
which  drinking  water  is  stored.  When  they  accumulate  in 
large  quantities  and  die,  they  cause  the  so-called  "  fishy 
taste  "  of  water.  This  trouble  has  been  to  some  extent  con- 
trolled during  recent  years  by  the  exclusion  of  light  from 
small  reservoirs,  and  by  the  addition  of  small  amounts  of 
copper  sulfate  to  the  water  in  large  reservoirs.  Copper  sul- 
fate  is  very  poisonous  to  algae,  even  in  quantities  of  one  part 
to  a  million  parts  of  water.  Since  animals  are  not  injured  by 
such  small  amounts,  the  water  may  be  used  without  harm  for 
drinking  purposes. 

The  seaweeds.  The  brown  and  red  algae,  commonly  known 
as  seaweeds,  grow  attached  to  rocks  in  the  shallow  water 
along  our  coasts.  They  are  generally  from  a  few  inches  to 
several  feet  in  length.  Some  of  the  brown  kelps  grow  in 
deep  water  and  attain  lengths  of  100  feet  or  more.  In  tex- 
ture the  seaweeds  vary  from  the  most  delicate  filaments  to 
broad,  leathery  expanses,  with  stalks  so  tough  that  they  are 
used  for  making  ropes. 

In  China  and  Japan,  seaweeds  have  long  been  used  for  food. 
They  are  cooked  with  fish  and  take  the  place  of  vegetables. 
Along  our  northern  coasts  the  "  Irish  moss  "  and  "  dulse  " 
are  collected  in  considerable  quantities  for  food  purposes.  The 
agar  used  in  laboratories  for  growing  bacteria  and  fungi  is 
made  from  certain  kinds  of  red  algae  that  grow  abundantly  on 
the  coasts  of  Asia. 

The  brown  kelps  are  an  important  source  of  iodin,  and  also 


The  Algge 


247 


FIG.  147.     Seaweeds  on  the  rocky  coast  of  Nova  Scotia. 


of  the  potassium  salts  which  are  required  in  large  quantities 
for  the  manufacture  of  commercial  fertilizers.  The  great  kelp 
beds  of  the  Pacific  coast  are  being  harvested  on  a  large  scale 
in  order  to  secure  these  products.  The  kelps  are  dried  and 
burned,  and  the  potassium  and  iodin  are  obtained  from  the 
ashes. 


Suggestions  for  Laboratory  Work  to  Precede  Chapter  Nineteen 

1.  Bacteria  and  molds  may  be  grown  for  study  by  placing  bread 
and  slices  of  boiled  potato  in  moist  chambers  (tumblers  inverted 
in  saucers). 

2.  Place  cultures  under  different  temperature  conditions  and 
note  the  effect  of  temperature  on  the  rate  of  growth.     Dry  a  part 
of  the  bread  and  potato  slices  somewhat,  and  compare  the  growth 
of  bacteria  and  fungi  on  the  dry  slices  with  the  growth  of  those  on 
the  moist  and  wet  slices.     What  do  these  experiments  suggest  re- 
garding methods  of  avoiding  bacteria  and  molds  ? 

3.  In  test  tubes,  yeast  may  be  grown  in  sugar  solutions  from 
commercial  yeast  cakes.     The  formation  of  carbon   dioxid  and 
alcohol  should  be  noted.     The  cells,  buds,  and  filaments  may  be 
seen  with  a  microscope. 

4.  From  grainfields  in   the  summer,  specimens  of  rusts  and 
smuts  can  be  obtained,  and  they  may  be  preserved  by  drying. 
The  cluster-cup  stage  of  rusts  is  common  on  elder,  crowfoot,  bar- 
berry, and  Indian  turnip.     In  the  laboratory  the  spores  may  be 
examined  and  injuries  to  the  leaves  noted. 

5.  Mushrooms  and  toadstools  may  be  collected  in  the  woods  at 
all  seasons.     Study  the  relation  of  the  fruiting  body  to  the  vegeta- 
tive part  of  the  plant.     Dried  material  and  specimens  preserved 
in  strong  alcohol  will  serve  to  show  the  variety  of  these  plants. 


248 


CHAPTER   NINETEEN 

BACTERIA   AND    FUNGI 

THE  seed  plants  and  algae  which  have  been  described  manu- 
facture their  own  food.  Both  of  them  are  independent  of 
other  plants  and  both  are  self-supporting.  They  are  called 
autophytes,  since  they  make  their  own  food  and  can  live 
without  the  assistance  of  other  plants.  There  are  many 
other  plants,  including  a  few  of  the  seed  plants,  which  lack 
chlorophyll  and  are  unable  to  manufacture  their  own  food. 
There  are  two  groups  of  these  dependent  plants,  saprophytes 
and  parasites. 

Saprophytes  live  on  organic  substances  which  have  been 
manufactured  by  other  plants  or  animals.  The  mold  that 
develops  on  bread  is  a  good  example  of  a  plant  of  this  kind. 
The  starch,  sugar,  and  protein  in  the  bread  were  made  orig- 
inally by  wheat  plants ;  and  the  mold,  therefore,  derives  its 
nourishment  from  substances  previously  manufactured  by  a 
green  plant.  It  grows  as  well  in  the  dark  as  in  the  light,  be- 
cause its  food  is  already  prepared.  It  needs  merely  to  digest 
the  food  and  absorb  it  into  its  own  body.  The  mushrooms 
are  mostly  saprophytes,  living  on  fallen  leaves,  tree  trunks, 
and  humus.  The  Indian  pipe  is  an  example  of  a  flowering 
plant  that  secures  its  food  in  the  same  way. 

Parasites  take  their  food  directly  from  other  living  plants, 
or  from  animals.  The  mistletoe  is  a  common  parasite  on 
various  trees  in  the  Southern  states.  Its  seeds  are  sticky  and 
adhere  to  the  bark  of  trees.  When  a  seed  germinates,  the 
hypocotyl  penetrates  the  bark  and  grows  inward  to  the 
cambium  and  the  food-conducting  tissue.  There  it  reaches 
the  food  made  by  the  tree  and  appropriates  some  of  it  to 
its  own  use.  The  plant  on  which  a  parasite  lives  is  called  its 

249 


250  Science  of  Plant  Life 

host.  The  yellow  dodder,  which  lacks  chlorophyll  altogether, 
is  a  common  parasite  related  to  the  morning-glory.  It 
twines  about  green  plants  and  sends  small  roots  into  them 
to  obtain  its  food.  Some  parasites  may  take  all  their  food 
from  the  host  plant,  as  does  the  dodder ;  others,  like  the 
mistletoe,  which  contain  some  chlorophyll,  may  secure  only 
a  part  of  their  nourishment  from  the  host. 

The  bacteria  and  fungi  form  a  great  group  of  simple  plants 
which  resemble  the  algae  in  structure,  but  which  differ  from 
the  algae  in  having  no  chlorophyll.  They  must,  therefore, 
live  as  saprophytes  or  parasites.  Either  they  grow  on  or  in 
living  plants  or  animals  and  draw  their  food  directly  from  them, 
or  they  feed  on  organic  matter. 

The  bacteria.  The  best  known  and  the  most  discussed  of 
all  the  simple  plants  are  the  bacteria.  They  are  so  intimately 
related  to  human  welfare  that  most  persons,  even  though  they 
have  never  seen  bacteria,  know  something  about  them.  Bac- 
teria  constitute  a  group  of  one-celled  plants,  at  once  the 
smallest  in  size,  the  simplest  in  structure,  and  the  most 
abundant  of  all  plants.  They  live  in  immense  numbers  in 
the  water  and  in  the  upper  layers  of  the  soil,  and  they  are 
blown  about  in  dust  in  the  air.  Some  are  too  small  to  be  seen 
except  with  the  highest  powers  of  the  microscope.  Others 
may  be  seen  with  an  ordinary  laboratory  microscope.  They 
make  up  for  the  small  size  of  the  individual  by  their  rapid 
multiplication,  and  by  the  formation  of  colonies  containing 
countless  numbers  of  individuals.  Bacteria  are  responsible 
for  many  of  the  diseases  of  men,  animals,  and  plants,  and 
bacteria  affect  our  lives  in  almost  countless  other  ways.  All 
our  modern  methods  of  sanitation,  quarantine,  surgery,  water 
supply  and  sewage  disposal,  and  much  of  our  personal 


Bacteria  and  Fungi 


251 


hygiene,  are  primarily  based  on  our  knowledge  of   the   be- 
havior of  this  group  of  plants. 


FIG.  148.     Various  forms  of  bacteria. 

Economically  the  bacteria  are  of  the  greatest  importance. 
Together  with  the  fungi  they  are  the  principal  cause  of  decay. 
Bacteria  bring  about  the  ripening  of  milk  in  butter  and 
cheese  making,  and  they  produce  both  the  pleasant  flavors 
in  these  products  and  the  unpleasant  flavors  that  develop  in 
them  with  age.  The  bacteria  are  also  the  source  of  much  of 
the  available  nitrogen  in  agricultural  soils  (page  32).  The 
drying  of  hay,  vegetables,  and  fruits,  the  canning  and  pickling 
of  vegetables,  fruits,  and  meats,  and  refrigeration  and  cold 
storage,  are  methods  of  avoiding  or  making  impossible  the 
growth  of  bacteria.  Thus  a  knowledge  of  these  plants  is 
fundamental  to  our  understanding  of  thousands  of  details 
of  our  daily  life. 

Growth  and  reproduction  of  bacteria.  Bacteria  grow  best 
and  reproduce  most  rapidly  in  the  presence  of  organic  food 
materials,  in  the  absence  of  light,  in  the  presence  of  moisture, 
and  at  moderately  high  temperatures  like  those  of  the  mid- 


252  Science  of  Plant  Life 

summer  months  and  of  our  own  bodies.  They  reproduce  by 
simple  cell  division,  the  cell  simply  pinching  in  two  to  form 
new  plants.  Many  forms  produce  spores,  similar  to  the  rest- 
ing spores  of  the  algae,  by  the  contraction  of  the  protoplasm 
in  the  cell  and  the  secretion  of  a  new  cell  wall.  Spores  are 
very  resistant  to  drying,  to  high  and  low  temperatures,  and 
to  poisons  which  readily  kill  the  ordinary  bacterial  cells.  It 
is  because  the  spores  of  certain  forms  withstand  the  temper- 
ature of  boiling  water  that  steam  pressure  is  used  in  sterilizing 
cans  of  corn,  beans,  peas,  and  other  vegetables.  Most  of 
the  common  disease-producing  bacteria,  however,  do  not  pro- 
duce spores. 

Bacteria  and  sanitation.  The  bacteria  of  decay  help  to 
keep  the  surface  of  the  earth  clean.  They  change  the  highly 
complex  organic  substances  that  form  the  bodies  of  plants 
and  animals  into  simple  substances  that  may  be  used  again 
by  other  plants  in  building  foods.  When  plants  and  animals 
die,  their  bodies  are  gradually  transformed  by  the  bacteria 
into  carbon  dioxid,  water,  and  mineral  salts.  The  sewage 
that  is  turned  into  our  rivers  is  chemically  changed  and  dis- 
posed of  in  the  same  way  by  these  minute  plants.  The  great 
increase  in  the  number  and  size  of  our  cities  has  made  it 
necessary  to  build  large  sewage-disposal  plants  where  the 
bacteria  can  act  more  rapidly  and  more  efficiently.  This 
prevents  the  pollution  of  streams  and  keeps  the  water  suitable 
for  city  water  supplies. 

The  modern  processes  of  filtering  and  sterilizing  the  water 
supplies  of  cities  are  carried  on  partly  to  remove  sediment 
and  partly  to  remove  disease-producing  bacteria.  Adding 
minute  quantities  of  alum  and  chloride  of  lime  to  the  water 
and  then  filtering  it  through  sand  not  only  renders  the  water 


Bacteria  and  Fungi  253 

clear,  but  removes  from  it  disease-producing  bacteria.  The 
most  dreaded  of  all  the  water-borne  diseases  is  typhoid  fever, 
and  the  cities  are  now  much  freer  from  this  disease  than  are 
the  country  districts  where  people  depend  upon  well  water. 
Surveys  in  some  of  the  Middle  Western  states  showed  that 
from  one  fifth  to  one  third  of  the  wells  examined  contained 
large  numbers  of  bacteria  derived  from  surface  drainage.  In 
such  wells  there  is  always  danger  that  the  surface  waters  may 
bring  in  disease-producing  bacteria,  especially  typhoid  germs 
derived  from  human  sources. 

Other  sanitary  practices,  such  as  quarantine,  disinfection, 
admitting  plenty  of  sunshine  into  living  rooms,  cleaning  walls 
and  floors,  removing  dust,  cooking  food,  washing  and  scald- 
ing dishes,  pasteurizing  milk,  and  keeping  food  supplies  in 
refrigerators,  are  all  related  to  the  control  or  elimination  of 
bacteria. 

Bacteria  and  disease.  When  certain  bacteria  grow  in  the 
body,  they  produce  poisonous  substances  called  toxins.  These 
toxins  interfere  with  the  normal  working  of  the  bodily  processes 
and  cause  illness.  The  body  under  these  circumstances  pro- 
duces substances  called  antitoxins,  which  protect  the  tissues 
until  the  bacteria  are  destroyed  by  leucocytes  (colorless  blood 
corpuscles)  or  in  other  ways.  Not  all  persons  are  equally 
susceptible  to  infectious  diseases.  A  person  may  be  immune 
to  a  disease  because  his  blood  contains  the  corresponding 
antitoxin  or  is  able  to  produce  it  or  because  his  blood  contains 
substances  that  kill  the  germs.  Habits  of  personal  cleanli- 
ness, sleeping  in  the  open  air,  and  careful  diet  help  to  increase 
immunity  against  many  of  the  common  diseases  to  which  we 
are  subject. 

Some  of  the  commoner  bacterial  diseases  are  tuberculosis, 


254 


Science  of  Plant  Life 


U.  S.  Dept.  of  Agriculture 

FIG.  149.     A  field  of  upland  cotton,  in  South  Carolina,  attacked  by  the  wilt  disease. 
The  fungus  that  causes  the  wilt  remains  in  the  soil  for  many  years, 

pneumonia,  grip,  diphtheria,  typhoid  fever,  colds,  lockjaw, 
and  blood  poisoning.  With  the  exception  of  lockjaw  and 
other  infections  through  wounds,  these  diseases  are  largely 
communicated  by  one  person  to  another.  Typhoid  is  carried 
also  by  means  of  water  and  milk  supplies  which  have  been 
contaminated,  and  by  flies  that  have  visited  infected  matter. 

A  fundamental  fact  that  should  be  learned  in  this  connec- 
tion is  that  no  one  can  contract  a  bacterial  disease  unless  he 
comes  in  contact  with  the  particular  bacterium  which  causes 
that  disease. 

The  natural  means  of  defense  against  disease  are  some- 
what similar  in  the  higher  plants  and  in  animals.  The  plant, 
in  addition  to  protective  chemical  substances  within  its  cells, 
has  an  epidermis  which  renders  the  entrance  of  bacteria  dif- 
ficult. Bacteria  are  able  to  enter,  however,  if  the  epidermis 
is  bruised  or  broken.  Plants  probably  suffer  from  bacterial 
diseases  as  much  as  do  animals.  Most  of  the  well-known 


Bacteria  and  Fungi 


255 


U.  S.  Dept.  of  Agriculture 

FIG.  150.     The  same  field  shown  in  Figure  140  planted  with  seeds  from  plants  that 
survived  the  attack  of  the  disease. 

plant  diseases,  however,  are  produced  by  fungi.  Of  the 
bacterial  diseases  of  plants,  the  twig  blight  of  pear  and  apple, 
the  cucumber  wilt,  and  the  crown  gall  of  various  plants  are 
perhaps  best  known. 

In  crop  plants  subject  to  disease,  it  has  been  found  possible 
in  some  instances  to  breed  varieties  that  are  immune  to  a 
particular  disease.  By  selecting  seed  from  plants  that  were 
least  affected,  or  not  at  all  injured,  in  a  diseased  field,  the 
characteristic  which  gave  that  plant  immunity  may  be  pre- 
served. In  this  way  wilt-resistant  cotton  was  bred  in  the 
South.  In  fields  where  the  wilt  disease  had  killed  nearly  all 
the  cotton,  it  was  noticed  that  an  occasional  plant  suffered 
scarcely  at  all.  Seedlings  grown  from  these  plants  were 
found  to  be  highly  resistant. 

By  hybridizing  with  related  species  or  varieties  that  are 
immune,  and  then  selecting  the  desired  offspring  for  producing 
seed,  it  is  possible  to  transfer  the  immunity  to  the  crop  plant 


256  Science  of  Plant  Life 

which  possessed  the  other  desired  qualities  but  which  was  not 
immune.  Such  desirable  combinations  are  possible  because 
most  of  the  qualities  of  plants  are  inherited  independently  of 
one  another.  For  example,  the  watermelon  wilt  threatened 
to  make  the  profitable  growing  of  watermelons  in  certain 
regions  along  the  Atlantic  coast  impossible.  It  was  dis- 
covered, however,  that  a  closely  related  plant,  a  small  African 
citron,  was  immune  to  the  disease.  The  problem  then  was 
to  cross  the  watermelon  with  the  citron  and  out  of  the  hybrid 
progeny  to  select  those  plants  that  possessed  all  the  good 
qualities  of  the  watermelon  and  the  immunity  of  the  citron. 
This  was  accomplished,  and  hybrid  watermelons  could  then 
be  grown  in  the  infected  fields. 

Bacteria  in  the  dairy.  Milk  is  an  ideal  medium  for  the 
growth  of  bacteria.  This  makes  necessary  the  most  careful 
handling  of  milk,  especially  when  it  is  used  directly  as  food. 
The  bacteria  get  into  the  milk  from  the  cow,  from  the  stable, 
from  the  vessels  into  which  the  milk  is  put,  and  from  the  persons 
who  handle  it.  Evidently  the  cows  should  be  kept  clean,  and 
the  stable  should  be  as  clean  and  free  from  dust  as  possible. 
The  vessels  with  which  the  milk  comes  in  contact  should  be 
sterile.  The  dairymen  should  have  clean  hands  and  clothes, 
and  above  all  they  should  be  free  from  infectious  diseases. 
Because  bacteria  multiply  very  rapidly  at  high  temperatures, 
the  milk  should  be  chilled  at  once  and  kept  on  ice.  To  make 
butter  and  cheese  of  fine  flavor,  pure  cultures  of  desirable  bac- 
teria are  added  to  the  milk  and  allowed  to  develop  for  a  time. 

In  order  to  avoid  the  danger  that  lies  in  the  use  of  milk 
contaminated  with  disease  germs,  milk  that  is  shipped  into 
the  larger  cities  is  usually  pasteurized  before  being  sold.  This 
treatment  kills  most  of  the  bacteria,  destroying  all  the  kinds 


Bacteria  and  Fungi  257 

that  produce  disease  in  human  beings.  By  "  pasteurization  " 
is  meant  the  heating  of  the  liquid  to  150  degrees  or  160  degrees 
F.  for  from  10  to  30  minutes.  This  does  not  kill  the  spores, 
but  they  are  to  a  large  extent  prevented  from  developing  by 
the  subsequent  cooling  that  the  milk  receives. 

The  preservation  of  foods.  The  greatest  losses  that  occur 
in  the  utilization  of  crops  are  connected  with  the  distribution 
of  the  products  to  the  consumer.  Much  of  the  food  pro- 
duced never  reaches  the  consumer,  because  bacteria  and 
molds  render  it  unfit  for  use  before  it  can  be  distributed 
through  the  markets.  There  are  four  methods  of  prevent- 
ing this  loss :  (i)  cold-storage  warehouses  and  refrigerator 
cars  are  used  to  keep  foods  below  the  temperature  at  which 
bacteria  grow  appreciably;  (2)  fruits,  vegetables,  or  other 
foods  are  packed  in  cans,  and  the  cans  are  then  sterilized  by 
heat  and  are  sealed  so  that  they  are  bacteria-tight ;  (3)  food 
products  are  dried  to  make  it  impossible  for  bacteria  to  grow 
in  them ;  and  (4)  foods  like  meat  and  fish  are  treated  with 
salt  or  with  some  other  chemical  that  will  prevent  the  growth 
of  bacteria.  Refrigeration  enables  us  to  preserve  foods  for 
weeks  and  months.  Canning  and  drying  make  foods  avail- 
able after  months  and  years. 

Soil  bacteria  and  humus.  In  the  process  by  which  the 
bacteria  of  decay  destroy  animal  and  vegetable  bodies,  the 
brown  organic  matter  represents  the  products  of  partial 
decomposition.  This  matter  is  called  humus,  and  it  is  one 
of  the  important  soil  factors  in  crop  production.  Humus  is 
of  importance  in  light  soils  because  it  helps  to  hold  the  water 
and  so  to  keep  the  soils  moist.  In  heavy  clay  it  mellows 
and  loosens  the  soil  and  promotes  aeration  and  drainage. 
Humus  may  also  contribute  something  directly  to  the  nu- 


258  Science  of  Plant  Life 

trition  of  the  crop,  for  among  the  multitude  of  substances 
it  contains  are  simple  carbon  and  nitrogen  compounds  which 
the  higher  plants  may  absorb  and  use  in  small  amounts. 

Soil  bacteria  and  nitrogen.  In  order  to  manufacture  pro- 
teins, seed  plants  must  have  a  supply  of  nitrogen.  This  they 
secure  principally  in  the  form  of  nitrates.  There  may  be 
other  nitrogen  compounds  in  the  soil,  but  they  are  unavail- 
able until  certain  nitrifying  bacteria  change  them  to  nitrates. 
Ammonia  is  one  of  the  nitrogen  compounds  produced  in  the 
process  of  humus  formation.  The  ammonia  may  be  acted 
upon  by  certain  bacteria  and  changed  to  nitrites,  which  in 
turn  are  changed  by  other  bacteria  into  nitrates.  These 
nitrates  are  used  by  the  green  plant.  The  nitrifying  bacteria 
are  of  great  importance  to  the  seed  plants,  because  they  break 
down  the  nitrogen  compounds  in  humus  and  other  soils,  and 
convert  the  nitrogen  into  an  available  form. 

Still  other  soil  bacteria  bring  about  a  process  known  as 
nitrogen  fixation,  by  which  nitrogen  is  actually  taken  from  the 
air  and  built  into  compounds  which  are  added  to  the  soil. 
The  nitrogen-fixing  bacteria  are,  with  a  few  exceptions,  the 
only  plants  that  can  take  nitrogen  from  the  air  and  combine 
it  to  form  nitrogen  compounds.  They  require  rich,  well- 
drained  soil  in  order  to  flourish.  They  are  of  great  importance 
in  agriculture  because  nitrogen  is  one  of  the  elements  that  is 
most  commonly  lacking  in  soils,  and  because  it  is  the  most 
expensive  of  all  the  elements  that  are  purchased  for  fertilizers. 

Bacteria  and  legumes.  Clover,  alfalfa,  beans,  soy  beans, 
and  peas  belong  to  a  family  of  plants  called  legumes.  They 
increase  the  nitrogen  in  soils  on  which  they  are  grown,  and 
for  many  years  they  have  been  used  in  crop  rotations,  fol- 
lowing wheat  or  corn.  The  practice  of  using  legumes  in 


Bacteria  and  Fungi 


259 


Bureau  of  Agriculture,  P.  /. 

FIG.  151.     A  field  of  cowpeas.     Like  other  legumes,  cowpeas  accumulate  nitrogen 
compounds,  and  when  plowed  under  they  add  nitrogen  and  humus  to  the  soil. 

crop  rotations  existed  long  before  the  real  cause  of  the  increase 
in  soil  nitrogen  was  understood,  or  even  before  it  was  under- 
stood how  different  elements  in  the  soil  contribute  to  its 
fertility.  By  experience  it  was  learned  that  other  plants 
flourish  in  land  after  leguminous  plants  have  been  grown  in 
it,  and  for  this  reason  the  farmer  included  legumes  in  his 
scheme  of  crop  rotation. 

It  is  now  clearly  understood  that  nitrogen  compounds  ac- 
cumulate in  leguminous  plants  only  because  of  the  presence 
of  certain  nitrogen-fixing  bacteria.  These  bacteria  occur  in 
many  soils,  and  when  the  legume  is  planted  and  develops  roots, 
they  invade  the  cells  of  the  root.  This  causes  the  infected 
parts  of  the  root  to  enlarge,  forming  nodules.  If  a  nodule 
from  a  clover  or  alfalfa  root  is  crushed  and  examined  under 


260 


Science  of  Plant  Life 


Bureau  of  Agriculture,  P.  I. 

FIG.  152.     A  field  of  young  bananas,  with  cowpeas  planted  between  the  rows  to 
enrich  the  soil. 


a  microscope,  it  will  be  found  to  be  filled  with  bacteria.  These 
bacteria  are  parasites  and  take  their  food  from  the  legume, 
but  by  the  processes  which  they  carry  on  in  the  nodules,  they 
change  nitrogen  from  the  soil  air  into  nitrogen  compounds, 
just  as  the  soil  bacteria  mentioned  in  the  preceding  section 
do.  The  nitrogen  compounds  thus  formed  are  used  by  the 
host  plant,  and  when  the  latter  is  plowed  under  and  decays, 
the  nitrogen  compounds  are  made  available  for  a  succeeding 
crop  of  wheat  or  corn. 

The  fungi.  The  fungi  form  an  exceedingly  large  and  di- 
versified group,  ranging  in  size  from  microscopic  forms  of  a 
single  cell  to  the  large,  fleshy  mushrooms  and  to  the  bracket 
fungi  found  on  tree  trunks  and  logs.  Among  the  most  impor- 
tant fungi  are  yeasts,  molds,  mildews,  smuts,  rusts,  and 
mushrooms.  We  cannot  here  describe  the  many  interesting 
forms  and  give  their  life  histories.  They  all  lack  chlorophyll, 


Bacteria  and  Fungi 


261 


however,  and  get  their  food  either  from  organic  matter  or 
from  living  plants.  The  plant  body  is  always  made  up  of 
filaments  like  those  of  the  algae.  In  the  higher  forms,  like  the 
mushrooms,  the  filaments  are  massed  together  into  a  com- 
pact, solid  structure. 

The  yeasts  and  molds,  and  most  of  the  mushrooms,  are 
saprophytes ;  the  smuts,  rusts,  and  some  of  the  mildews  are 
parasites  upon  the  higher  plants,  and  they  cause  serious  losses 
to  the  farmer  and  gardener.  A  few  of  the  mushrooms  are  edible 
and  furnish  small  quantities  of  food  for  animals  and  man. 

The  yeasts.  In  the  making  of  bread,  yeasts  are  of  primary 
importance.  They  are  small,  one-celled  plants  that  multi- 
ply very  rapidly,  and  when  properly  mixed  with  flour  and 
water  they  develop  in  all  parts  of  the  dough.  The  yeasts 
have  within  them  enzymes  which  change  part  of  the  sugar 
that  is  present  into  carbon  dioxid  and  alcohol.  The  carbon 
dioxid  accumulates  in  bubbles  and  makes  the  dough  light. 
When  the  dough  is  put  into  a  hot  oven  the  alcohol  is  vaporized, 
and  together  with  the  carbon  dioxid  it  is  driven  off  into  the 
air.  The  high  temperature  kills  the  yeast,  bakes  the  dough, 
and  changes  some  of  the  starch 
into  its  soluble  form,  dextrin, 
which  makes  it  more  readily 
digestible.  Sour  bread  is  pro- 
duced when  the  yeast  that  is 
added  contains  acid-forming 
bacteria  which  change  part  of 
the  alcohol  into  acetic 'acid.  (j-^~&  &  ^® 

Yeasts  and  bacteria  are  the  ®       ^ 

Organisms     that     Change     fruit    ^  I53'     Yeast  cells  and  chains  of  cells. 

Above  are  three  cells,  each  containing  four 

juice  into  cider  and  vinegar,  resting  spores. 


262 


Science  of  Plant  Life 


Yeast  first  changes  the  sugar  in  apple  juice  to  carbon  dioxid 
and  alcohol,  and  bacteria  further  oxidize  the  alcohol  to  acetic 
acid  or  vinegar.  Yeasts  are  also  used  in  the  fermenting  of 
beer  and  wines. 

Yeast  may  readily  be  grown  by  adding  a  bit  of  yeast  to  a 
5  per  cent  sugar  solution  in  a  test  tube.  The  branching  groups 
of  cells  may  then  be  examined  under  the  microscope.  The 
manner  of  forming  new  cells  among  yeasts  is  unique,  in  that  the 
new  cells  start  as  small  protuberances  (buds)  from  the  older 
cells.  These  buds  gradually  enlarge  until  they  attain  their 
complete  growth  and  separate.  The  alcohol  formed  in  the 
test  tube  by  the  yeast  may  easily  be  detected  by  its  odor. 

The  molds.  The  molds  are  usually  white,  filamentous 
plants  that  are  of  great  economic  importance  because  of  the 
damage  that  they  do  to  foods  during  storage  or  shipment. 
Like  bacteria,  the  spores  of  molds  are  in  the  air  and  in  the 
dust  everywhere.  If  the  temperature  is  warm  and  the 
food  is  moist,  they  germinate  and,  together  with  bacteria,  soon 


FIG.  154.  Bread  mold,  showing  the  rhizoids  which  penetrate  the  material  on  which 
the  mold  grows  and  which  absorb  food,  the  horizontal  filaments  by  which  it  spreads, 
and  the  vertical  filaments  which  bear  the  sporangia  and  spores. 


Bacteria  and  Fungi  263 

destroy  food.  The  same  measures  that  will  prevent  the 
growth  of  bacteria  in  foods  will  prevent  the  growth  of  the 
molds,  which  are  usually  associated  with  them. 

The  molds  exemplify  one  of  the  fundamental  characteristics 
of  the  fungi ;  namely,  their  capacity  for  producing  enormous 
numbers  of  spores.  In  some  cases,  as  in  the  bread  mold,  these 
spores  are  produced  in  rounded  sacs  called  sporangia;  in  other 
cases  upright  filaments  of  the  mold  develop  spores  by  cutting 
off  chains  of  little  rounded  cells  from  the  ends  of  the  filaments. 
These  spores  are  of  various  colors,  —  brown,  black,  blue,  green, 
or  yellow,  —  and  they  give  the  characteristic  color  to  the 
mold.  In  most  molds  the  spores  begin  to  develop  at  ordinary 
temperatures  within  2  or  3  days  after  the  parent  spore  germi- 
nates. 

The  rusts.  Among  the  most  serious  diseases  affecting 
wheat,  rye,  barley,  and  oats  are  those  produced  by  the  fungi 
known  as  the  rusts.  These  fungi  are  called  rusts  because 
plants  that  are  infected  with  them  develop  yellow  and  brown 
spots  that  have  the  appearance  of  iron  rust.  The  rusts  occur 
wherever  grains  are  grown,  and  they  cause  millions  of  dol- 
lars' worth  of  damage  to  crops  every  year. 

The  rusts  are  parasites  that  live  inside  the  host  plants  and 
injure  or  destroy  the  tissues  which  are  concerned  in  food  manu- 
facture. Their  life  history  is  peculiar  in  that  the  fungus  usually 
produces  diseases  on  two  different  kinds  of  host  plants.  The 
stem  rust  of  wheat,  for  example,  produces  patches  of  red 
spores  which  will  infect  other  wheat  plants.  It  produces  also 
black  spores  which  live  over  winter  on  the  stubble,  and 
which  germinate  the  following  spring  and  produce  a  third  kind 
of  spore  that  infects  the  barberry.  On  the  barberry  leaves 
the  fungus  produces  a  cuplike  depression  within  which  a  fourth 


264 


Science  of  Plant  Life 


kind  of  spore  is  formed.     This  spore  will  not  germinate  on 
the  barberry,  but  it  will  infect  wheat.     Thus  the  stem  rust 


FIG.  155.  Life  history  of  the  stem  rust  of  wheat.  In  fields  where  wheat  has  been 
grown,  the  stubble  (A)  carries  over  the  winter  black  spores  (B),  that  germinate  in  early 
spring,  producing  smaller  spores  (C).  These  infect  the  leaves  of  the  common  bar- 
berry (D).  In  the  leaves  of  the  barberry  the  fungus  grows  and  produces  cup-shaped 
cavities  filled  with  spores  (£)  that  are  carried  by  the  wind  to  wheatfields  and  infect 
the  wheat  plants.  After  growing  in  the  wheat  a  short  time,  the  fungus  produces  first 
the  red  spores  (G)  that  spread  the  disease  to  other  wheat  plants,  and  later  the  two-celled 
black  spores  that  carry  the  disease  over  the  winter  again. 

of  wheat  spreads  from  one  wheat  plant  to  another  by  means 
of  red  spores,  from  wheat  to  the  barberry  by  spores  that  are 
produced  the  next  spring,  and  from  the  barberry  back  to  the 
wheat  by  still  another  kind  of  spore. 

In  the  Northern  states,  from  the  Dakotas  to  New  England, 
the  barberry  stage  is  of  special  importance  in  the  life  history 
of  the  rust.  In  the  central  United  States  where  winter 
wheat  is  grown,  the  red  spores  produced  during  the  summer 
drop  to  the  ground  and  infect  the  wheat  planted  in  the  au- 
tumn. In  this  way  the  rust  may  be  perpetuated  from  year 


Bacteria  and  Fungi 


265 


to   year   without   the   intervening   barberry   stage.     In   the 
Northern  states   the  destruction  of  all  barberry  plants  has 


FIG.  156.  The  white  pine  blister  rust.  The  fruiting  bodies  on  the  white  pine  (A) 
produce  spores  that  infect  the  leaves  of  the  gooseberry  (B  and  C).  On  the  gooseberry 
leaves  the  fungus  produces  at  first  yellow  spores  that  will  infect  other  gooseberry  plants, 
and  later  brown  spores  that  carry  the  disease  back  to  the  pine.  When  a  pine  (D)  is 
infected  by  the  disease,  the  younger  parts  soon  die  (£). 

been  undertaken,  and  this  work  will  doubtless  reduce  the 
amount  of  infection.  The  hope  of  effectively  controlling 
wheat  rust,  however,  probably  lies  in  breeding  new  varieties 
of  wheat  that  are  immune  to  the  disease. 

Other  rusts  also  live  on  two  host  plants,  and  because  of 
this  double  life  and  the  fact  that  the  fungus  grows  on  the 
inside  of  its  host,  they  are  very  difficult  to  control.  The  rust 
on  the  red  cedar  produces  the  so-called  "  cedar  apples,"  the 
spores  from  which  infect  the  leaves  of  the  apple  tree  and  may 
do  great  damage  to  them.  Recently  the  blister  rust  of  the 
white  pine  has  been  brought  to  America,  and  it  threatens  to 
destroy  what  remains  of  our  white-pine  forests.  In  this  case 


266 


Science  of  Plant  Life 


the  alternate  host  plants  are  the  wild  and  cultivated  goose- 
berries and  currants.  Another  common  rust  is  frequently  seen 
on  raspberry  and  blackberry  bushes  along 
roadsides  ;  it  colors  the  under  sides  of  the 
leaves  with  its  bright,  orange-red  spores. 
The  smuts.  The  smuts  of  oats,  wheat, 
barley,  and  corn  often  greatly  reduce  the 
yield  of  these  plants.  But  since  their 
life  histories  are  known,  it  is  compara- 
tively easy  to  control  them.  The  smut 
fungi  generally  are  carried  over  from  one 
year  to  the  next,  on  or  in  the  grain,  and 
they  may  live  over  winter  in  the  soil. 
When  the  grain  is  planted,  the  smut 
spores  germinate  and  infect  the  young 
seedlings.  The  smut  plant  lives  inside 
the  host,  and  its  presence  becomes  ap- 
parent only  when  the  black  spores  of 
the  fungus  appear,  usually  in  the  grain. 
FIG.  157.  A  normal  and  In  some  cases  the  walls  of  the  grain  are 
smutted  flower  cluster  of  destroyed  and  the  spores  are  scattered 

oats. 

by  the  wind. 

All  smuts  of  the  small  grains  may  be  prevented  from 
germinating  by  soaking  the  seed  in  hot  water  for  a  short  time 
before  planting.  It  is  easier,  however,  to  wash  the  grain  with 
a  weak  solution  of  formalin,  and  this  treatment  is  effective 
in  preventing  the  growth  of  those  smuts  whose  spores  are 
carried  over  the  winter  on  seed.  Detailed  information  con- 
cerning seed  treatment  for  the  prevention  of  the  several 
kinds  of  smuts  may  be  obtained  from  State  Agricultural  Ex- 
periment Stations.  The  corn  smut  is  controlled  by  removing 


Bacteria  and  Fungi 


267 


FIG.  158.  Various  forms  of  mushrooms.  At  top :  puffballs  in  center,  bracket 
fungus  (Fames)  at  right,  Hydnums  to  left.  Center :  coral  fungus  (Clavaria) 
on  left  and  poisonous  Amanita  on  right.  At  bottom,  left  to  right:  cornu- 
copia fungus  (Craterelhis),Russula,znd  earthstars  (Geaster). 


268 


Science  of  Plant  Life 


and  burning  the  plants  as  soon  as  evidence  of  the  disease 
appears. 

Mushrooms  and  toadstools.  The  largest  and  most  com- 
plex of  the  fungi  are  the  mushrooms  and  toadstools.  They 
are  common  in  fields  and  woods  and  for  the  most  part  live  on 
decaying  wood  and  on  humus  in  the  soil.  There  is  no  real 
distinction  between  mushrooms  and  toadstools.  Some  of 
them  are  edible,  others  are  indigestible,  and  some  are  deadly 
poisonous.  Edible  forms  are  cultivated  on  a  large  scale  in 
caves  and  abandoned  mines,  and  on  a  smaller  scale  in  cellars. 
Wild  forms  should  not  be  eaten  unless  they  are  gathered  by 
persons  competent  to  distinguish  the  different  species,  many 
of  which  are  similar  in  appearance  but  very  different  in  their 
effects  when  eaten. 

The  mushrooms  as  they  are  gathered  are  only  the  fruiting 
bodies  of  the  fungi.  The  real  plant  consists  of  bundles  of 
filaments  extending  in  all  directions  throughout  a  large  mass 


FIG.  159.     Stages  in  the  development  of  the  common  edible  pink-gilled  mush- 
room.   Note  the  underground  vegetative  body  of  the  plant. 


Bacteria  and  Fungi 


269 


of  soil  on  which  the  fruiting  bodies  appear.  It  may  take 
several  years  for  the  underground  vegetative  part  of  the  fungus 
to  develop,  while  the  fruiting  bodies  may  develop  in  a  few 
days.  It  is  the  enlargement  of  the  fruiting  bodies  that  per- 
sons have  in  mind  when  they  speak  of  "  mushroom  growth." 
This  expression  leaves  out  of  account  the  months  or  years  of 
growth  during  which  the  materials  were  accumulated  for  the 
sudden  production  of  the  fruiting  body.  The  spores  of  mush- 
rooms are  produced  in  unthinkable  numbers  either  inside  the 
fruiting  body  (puffballs),  on  the  upper  surface  (morels),  or 
on  the  under  side  of  the  umbrella-shaped  cap  (mushrooms) 

(Fig-  159)-' 

Lichens.  Among  the  parasitic  fungi  are  some  that  live  on 
such  one-celled  algae  as  Protococcus.  The  fungus  forms  the 
plant  body,  and  the  algal  cells  are  completely  enveloped. 
These  forms  constitute  the  lichens,  which  are  gray-green,  ir- 
regular-shaped plants  that  are  common  on  the  bark  of  trees, 


FIG.  160.     Several  common  lichens.     The  form  on  the  tree  is  Parmelia ;  the  one  in 
the  middle  foreground  is  Peltigera;  the  other  forms  are  species  of  Cladonia. 


270  Science  of  Plant  Life 

on  rock  surfaces,  and  occasionally  on  the  soil  (Fig.  160).  Like 
other  fungi  they  produce  fruiting  bodies,  —  small  cup-shaped 
or  disklike  elevations,  —  in  which  asexual  spores  are  pro- 
duced in  great  numbers. 

Summary  6f  the  simple  plants.  The  simplest  forms  of 
plant  life  include  three  great  groups  that  are  of  the  highest 
importance  to  man : 

(1)  The  algae  constitute  the  primary  food  of  fishes,  and 
they  will  become  increasingly  important  as  the  cultivation 
of  ponds,  lakes,  and  streams  (aquaculture)  for  the  production 
of  fish  becomes  more  necessary  to  augment  our  food  supplies. 

(2)  The  bacteria  have  a  most  important  and  intimate  bear- 
ing upon  the  lives  of  all  of  us.     They  have  made  necessary 
our  various  food-preserving  industries.     Their  existence  every- 
where is  the  chief  factor  that  must  be  considered  in  personal 
hygiene  and  in  the  making  of  our  sanitary  laws.     Some  kinds 
of  bacteria  aid  other  forms  of  life  by  destroying  the  bodies  of 
dead  organisms  and  improving  the  fertility  of  soils ;    other 
bacteria  are  responsible  for  many  diseases  of  both  plants  and 
animals. 

(3)  The  fungi  also  are  destructive  agents,  causing  injury 
to  our  crop  plants,  the  loss  of  much  valuable  food,  and  the 
destruction  of  timber.     Forms  like  the  yeasts  are  valuable 
aids  in  certain  food  industries.     A  very  few  of  the  fungi  are 
themselves  sources  of  food.     The  fungi  are  notable  for  the 
enormous  number  of  spores  which  they  produce. 

Algae  are  autophytes,  while  the  bacteria  and  fungi  are 
saprophytes  or  parasites.  The  algae  are  of  world- wide  dis- 
tribution, but  they  are  chiefly  confined  to  aquatic  and  marine 
habitats.  The  bacteria  and  fungi  are  also  world- wide  in  their 
distribution,  but  they  are  most  numerous  on  land. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Twenty 

1.  Make  a  field  study  of  liverworts  and  mosses,  noting  especially 
conditions  most  favorable  to  them.     Examine  underground  parts 
as  well  as  above-ground  parts. 

In  winter  a  visit  to  a  conservatory  or  greenhouse  will  enable 
students  to  see  these  forms  in  a  growing  condition. 

2.  Specimens  of  liverworts  may  be  secured  in  spring  from  moist 
rocks  along  streams,  from  the  soil  in  clover  fields,  and  from  trees 
in  moist  woods.     In  Marchantia,  study  particularly  the  thallus, 
rhizoids,    growing    region,    cupules,    gemmae,    and    reproductive 
branches. 

3.  Of  the  mosses,  Mnium  and  Polytrichum  are  particularly 
good  to  show  protomena,  rhizoids,  stem,  leaves,  and  reproductive 
structures.     Compare  the  vegetative  plant  body  of  a  moss  with 
that  of  a  liverwort  in  its  relation  to  water,  light,  and  air. 


271 


CHAPTER   TWENTY 

LIVERWORTS   AND    MOSSES 


FIG.  161.    A  common  liverwort  (M archantid) ,  showing  thallus,  cupules,   and 
reproductive  branches. 

THE  largest  of  the  mosses  and  liverworts  never  attain  a 
height  or  length  of  more  than  a  few  inches,  and  they  are  of 
very  simple  structure  in  comparison  with  the  flowering  plants. 
In  contrast  with  the  algae,  which  on  the  whole  are  water 
plants,  mosses  and  liverworts  for  the  most  part  live  on  land. 
The  passing  of  plants  from  a  water  to  a  land  habitat  is  one  of 
the  notable  steps  in  the  development  of  the  plant  kingdom, 
and  in  connection  with  the  study  of  this  group  we  shall  con- 
trast the  environments  of  land  and  water  plants  and  consider 
the  modifications  in  structure  that  accompany  the  passing  of 
plants  from  the  water  to  a  land  habitat. 

The  liverworts.  The  body  df  a  liverwort  is  flat  and  leaflike 
and  is  called  a  thallus  (plural,  thalli).  It  may  be  from  one  to 
several  cell  layers  in  thickness.  It  grows  at  the  tip,  and 
usually  branches  by  forking  at  intervals.  Liverworts  do  not 

272 


Liverworts  and  Mosses  273 

stand  erect,  but  usually  have  their  thalli  in  close  contact 
with  the  substrata  on  which  they  grow.  In  most  forms  the 
thallus  is  a  continuous  plate  of  cells,  but  some  forms  have 
prostrate  stems  with  small  leaves  on  either  side.  All  of  them 
have  chlorophyll  and  manufacture  their  own  food.  All  the 
forms  have  on  their  under  surfaces  small,  hair  like  rhizoids 
that  anchor  the  plant  and  absorb  water  and  minerals. 

Liverworts  are  widely  distributed  but  are  most  numerous 
in  the  tropics.  A  few  of  them  are  found  on  trees  and  rocks 
and  a  few  are  found  floating  on  water,  but  as  a  whole  they 
live  on  moist  soil  and  in  shaded  situations. 

Reproduction  among  the  liverworts  takes  place  by  means 
of  spores,  produced  either  directly  on  the  thallus  or  on  special 
branches.  In  many  liverworts  there  are  produced  also  special 
bodies  called  gemma,  (singular,  gemma),  which  propagate  the 
plants  vegetatively. 

The  liverworts  are  supposed  to  be  descended  from  plants 
like  the  green  algae,  for  it  is  thought  that  the  simplest  plants 
existed  first  and  that  plant  life  as  well  as  animal  life  had  its 
origin  in  the  water.  The  liverworts  may  be  considered,  there- 
fore, as  a  group  of  simple  plants  that  exhibit  some  of  the  stages 
through  which  plants  pass  in  going  from  the  water  and  taking 
up  their  life  upon  the  land.  In  this  respect  they  can  be  com- 
pared to  the  amphibious  (Greek :  amphi,  double,  and  bios, 
life  ;  i.e.,  life  both  on  land  and  in  water)  frogs  and  salamanders 
of  the  animal  world.  Just  how  a  plant  becomes  adjusted  to 
its  new  environment,  when  it  is  forced  to  live  on  land  because 
of  the  drying  up  of  the  pool  in  which  it  grew,  is  a  most  inter- 
esting question  to  consider. 

Living  conditions  of  water  and  land  plants  contrasted. 
The  algae  probably  represent  the  remnants  and  derivatives 


274  Science  of  Plant  Life 

of  the  first  plants  that  grew  on  the  earth,  and  the  shallow 
water  in  which  the  fresh-water  algae  live  is  the  most  favorable 
of  all  habitats  for  plants.  Because  the  plants  are  of  about 
the  same  weight  as  water  or  are  lighter,  they  are  supported  by 
the  water  and  do  not  need  to  use  their  foods  for  building 
mechanical  tissues.  In  the  shallow  water  there  is  sufficient 
light  for  photosynthesis,  while  the  plants  are  protected  from 
the  heating  and  drying  effects  of  the  intense  sunshine  to  which 
land  plants  in  many  situations  are  exposed.  An  abundant 
supply  of  the  carbon  dioxid  and  oxygen  needed  for  photo- 
synthesis and  respiration  is  in  solution  in  the  water,  and 
mineral  salts  sufficient  for  the  needs  of  the  plants  are  washed 
in  with  every  rain.  Furthermore,  the  temperature  is  more 
uniform  than  on  land,  and  this  permits  the  processes  of  growth 
and  reproduction  to  go  on  almost  uninterruptedly  throughout 
the  entire  twenty-four  hours.  The  length  of  the  growing 
season  in  temperate  and  cold  climates  is  longer  under  the 
water  than  on  land,  because  the  water  protects  the  plants  from 
the  sudden  changes  of  temperature  to  which  land  plants  are 
subjected  in  the  spring  and  fall.  The  shallow-water  plants, 
therefore,  live  under  the  conditions  most  favorable  for  plant 
life.  Even  the  deep-water  plants  like  some  of  the  marine  algae, 
although  they  are  under  the  great  disadvantage  of  having 
little  light,  gain  many  advantages  through  their  water  en- 
vironment. 

The  environment  of  the  land  plant  provides  a  supply  of 
carbon  dioxid  and  oxygen  directly  from  the  atmosphere,  and 
mineral  salts  may  be  secured  from  the  soil  water  with  which 
the  plants  are  in  contact.  But  if  the  plant  grows  in  full 
sunlight,  it  is  subjected  to  much  more  intense  illumination  and 
heating  than  are  water  plants,  and  it  must  withstand  the 


Liverworts  and  Mosses 


275 


drying  effects  of  the  air.     In  aquatic  plants  the  cells  are  never 
without  an  adequate  supply  of  water,  while  in  land  plants 


FIG.  162.  Cross  section  of  Marchantia  thallus,  showing  rhizoid  (below),  water- 
storage  tissue,  the  air  chambers  containing  the  principal  photosynthetic  cells,  and 
the  epidermis  which  forms  a  transparent  roof  over  the  air  chambers.  Starch 
grains  occur  in  many  of  the  cells  of  the  water-storage  tissue. 

transpiration  may  reduce  the  water  content  of  the  cells  to 
such  an  extent  that  they  may  be  injured  or  even  die.  A 
study  of  the  amphibious  liverworts  shows  that  they  have 
become  adjusted  only  to  a  medium  light  and  a  moderate 
amount  of  drying.  These  plants,  therefore,  grow  in  shaded 
and  moist  situations.  During  wet  periods  many  individuals 
start  in  intensely  illuminated  places,  only  to  be  killed  off 
by  the  light  and  its  secondary  temperature  and  drought  ef- 
fects. The  shaded  situation  where  the  water  is  near  the 
surface  of  the  soil  is  evidently  the  habitat  where  these  plants 
suffer  the  least,  and  this  explains  why  liverworts  persist  in 
moist  situations  and  not  in  the  open. 

Responses  of  the  plant  to  the  aerial  environment.  The 
land  liverworts  show  several  changes  in  structure  that  are 
of  advantage  to  the  plants  in  an  aerial  life.  The  more  im- 
portant of  them  are : 


276  Science  of  Plant  Life 

(1)  Firmer,  and  in  some  cases  thicker,  cell  walls  and  water- 
storage   tissue.     The  firmer  cell  walls  are  less  permeable   to 
water  and  reduce  the  rate  of  water  loss.     Furthermore,  the 
plants  grow  flat  on  the  soil  in  contact  with  the  water  supply, 
and  some  of  the  forms  develop  layers  of  water-storage  cells 
on  the  side  in  contact  with  the  soil  (Fig.  162).     This  enables 
them  to  withstand  short  dry  periods  better  than  do  those 
forms  that  have  only  the  ordinary  green  cells.     The  develop- 
ment of  firmer  cell  walls  and  water-storage  tissue  is  the  first  ad- 
justment to  land  conditions. 

(2)  The  development  of  rhizoids.     Land  plants  are  liable 
to  be  washed  away  by  rain  and  surface  water,  and  on  this 
account  they  need  some  anchorage ;   also,  it  is  necessary  for 
them  to  have  structures  that  will  bring  them  into  contact  with 
the   soil-water   supply.     In   the   liverworts,    rhizoids   anchor 
the  plant  and  to  some  extent  absorb  water  and  mineral  salts 
from  the  soil.     Rhizoids  are  elongated  cells  that  develop  on 
the  lower  side  of  the  plant  body  and  penetrate  the  soil.     They 
resemble  root  hairs  in  form.     The  development  of  rhizoids, 
therefore,  represents  a  second  important  adjustment  of  plants  to 
the  land  environment. 

(3)  The    development    of    an   epidermis.     The   land   liver- 
worts are  covered  by  an  epidermis  which  helps  to  protect 
them  against  water  losses.     Since  a  ready  access  to  carbon 
dioxid  and  oxygen  is  necessary  for  photosynthesis  and  res- 
piration, the  liverworts  with  thicker  bodies  have  openings  or 
pores  in  the  epidermis.     Through  these  pores  the  gas  ex- 
changes between  the  air  and  the  cells  within  the  body  can 
take  place.     In  the  more  complex  liverworts,  the  epidermis 
is  raised  like  a  transparent  roof  on  ridges  of  supporting  tissues, 
leaving  beneath  it  a  series  of  small  air  chambers  in  which  the 


Liverworts  and  Mosses  277 

chlorophyll-bearing  cells  stand  up  in  short  chains  (Fig.  162). 
Each  chamber  is  connected  with  the  air  by  a  pore  in  the 
center  of  its  roof.  In  these  rather  simple  plants,  therefore, 
protection  against  water  loss  is  accomplished  in  much  the 
same  way  as  in  seed  plants,  but  the  epidermal  pores  in  them 
are  chimney-like  openings  and  are  incapable  of  closing  as  do 
the  stomata  of  the  higher  plants.  The  development  of  an 
epidermis  and  of  pores  is  a  third  adjustment  of  plants  to  the 
land  environment. 

(4)  The  ability  to  withstand  drying.  When  the  cells  of 
water  plants  are  dried,  the  protoplasm  dies  at  once;  but  a 
few  of  the  liverworts,  like  many  mosses  and  like  Protococcus 
and  a  few  other  algae,  do  not  die  when  water  is  lost  from  the 
cells.  Just  what  quality  the  protoplasm  possesses  that  en- 
ables it  to  withstand  drying,  it  is  impossible  to  say ;  but  some 
of  the  liverworts  that  grow  on  trees  and  rocks  possess  this 
quality,  and  certain  mosses  have  to  a  remarkable  degree  the 
ability  to  withstand  drying.  A  fourth  adjustment  of  plants 
to  the  land  environment  is  the  development  of  the  ability  to  with- 
stand drying. 

The  mosses.  The  mosses  form  a  very  large  group  found 
in  all  parts  of  the  world.  They  usually  have  upright  stems, 
though  many  live  close  to  the  substratum  and  have  only 
horizontal  or  inclined  stems.  They  possess  very  simple 
leaves,  frequently  only  one  cell  layer  in  thickness,  sometimes 
thicker  toward  the  midrib. 

The  mosses,  like  the  liverworts,  are  most  abundant  in  moist, 
partly  shaded  habitats.  A  few,  however,  grow  on  rocks  and 
trees  where  they  are  exposed  to  periodic  drought.  These 
latter  forms,  like  Protococcus,  have  the  power  to  with- 
stand complete  drying.  When  dry,  they  are  in  a  dormant 


278 


Science  of  Plant  Life 


FIG.  163.    Mosses,  showing  the  compact  grouping  of  the  plants. 

condition;  and  when  wet,  they  go  on  with  their  normal 
processes  of  photosynthesis,  respiration,  growth,  and  repro- 
duction. 

Mosses  also  possess  means  of  anchorage  by  rhizoids.  The 
rhizoids  of  the  liverworts  are  one-celled  structures.  Those  of 
the  mosses  are  branching,  many-celled  structures  which  pene- 
trate the  soil,  affording  a  firm  hold  and  absorbing  a  part  of 
the  water  used  by  the  plant.  The  habit  of  growing  in  com- 
pact clusters  gives  the  mosses  a  method  of  conserving  water 
and  maintaining  the  water  balance,  other  than  the  methods 
spoken  of  in  connection  with  the  liverworts ;  the  dense  masses 
of  plants  take  up  water  from  rains  and  hold  it  for  some  time 
like  a  sponge. 

Mosses,  therefore,  show  some  advances  over  the  liverworts 
in  their  upright  stems  and  branching  rhizoids,  in  the  regular 
occurrence  of  simple  leaves,  and  in  their  ability  to  grow  in 
drier  habitats.  The  liverworts  and  mosses  together  show  the 


Liverworts  and  Mosses  279 

successful  adjustment  of  simple  green  plants  to  the  land 
environment. 

Life  history  of  the  moss.  Mosses  reproduce  by  vegetative 
propagation  and  by  both 
sexual  and  asexual  spores. 
A  study  of  each  of  these 
methods  will  make  clear  the 
somewhat  complicated  life 
history  of  the  moss  plant. 

Vegetative     multiplication. 

When    a    mOSS    Spore    germi-     FIG.  164.     Moss  spores  (A)  and  protonema 

nates  on  the  soil,  it  produces    (B)'  with  f  bud  (C)  from  which  an  upright 

stem  develops. 

a     branching,     filamentous 

body,  the  protonema,  which  resembles  some  of  the  branching 
forms  among  the  green  algae.  The  protonema  spreads  over 
the  soil  for  some  distance  and  then  develops  numerous  buds 
(Fig.  164).  The  buds  give  rise  to  the  upright  leafy  branches 
which  we  commonly  call  the  moss  plant.  Because  of  the 
numerous  buds  developed  on  the  protonema,  the  moss  plants 
stand  in  thick  clusters  or  masses. 

The  upright  leafy  stems  of  the  moss  also  have  the  power 
of  producing  protonema-like  branches  which  spread  still 
farther  over  the  soil,  thus  serving  to  multiply  the  plants  and 
to  make  the  plant  mass  denser  and  larger.  In  some  mosses 
with  horizontal  or  inclined  stems,  the  stem  tips  when  in  con- 
tact with  the  soil  develop  rhizoids  and  give  rise  to  new  branches, 
much  as  the  stems  of  the  raspberry  develop  new  plants  (page 
217).  These  methods  of  vegetative  propagation  are  com- 
mon among  the  mosses,  and  some  mosses  are  not  known  to 
multiply  in  any  other  way. 

Sexual  reproduction.     The  upright  stems  of  most  mosses 


280 


Science  of  Plant  Life 


when  mature  produce  gametes  —  egg  and  sperm  cells  —  in 
special  organs  at  the  stem  tips.     The  sperms  are  swimming 


FIG.  165.  A  moss  plant  (Mnium).  £  is  a  vegetative  branch,  B  a  branch  that  produces 
eggs,  and  A  a  branch  that  produces  sperms.  After  fertilization,  an  upright  stalk  bear- 
ing a  spore  case  (C)  develops  from  the  egg.  A '  is  a  longitudinal  section  of  a  female 
branch,  showing  three  egg  cells  in  the  cases  in  which  they  are  produced ;  B'  is  a  section 
of  a  male  branch,  showing  three  of  the  organs  that  produce  the  sperms. 

cells  much  like  those  of  algae.  As  they  cannot  reach  the  egg 
cells  except  by  swimming,  fertilization  takes  place  only  when 
the  moss  is  wet.  When  the  sperm  unites  with  the  egg,  it 
forms  an  oospore. 

Asexual  reproduction.  The  oospore  germinates  while  still 
on  top  of  the  parent  stem,  and  produces  a  long,  stalklike  body. 
The  base  of  this  body  grows  downward  into  the  parent  stem 
and  draws  water  and  nourishment  from  it.  At  the  top  of 
the  stalk  a  sporangium  or  capsule  develops  which  contains 
asexual  spores.  The  stalk  and  sporangium  live  parasitically 
on  the  green,  leafy  moss  plant  and  are  a  distinct  stage  in  the 
life  of  the  plant. 


Liverworts  and  Mosses  .    281 

The  first  stage  in  the  life  history  of  the  plant  ends  with 
the  production  of  oospores.  The  second  stage  ends  with  the 
formation  of  asexual  spores.  When  the  asexual  spores  germi- 
nate, they  again  produce  the  protonema  and  the  leafy  moss 
plants.  The  asexual  spores  are  small,  firm-walled,  rounded 
bodies  that  are  able  to  withstand  the  drying  effects  of  the 
air.  They  are  fitted  to  be  blown  about,  and  so  to  start  the 
growth  of  the  plant  in  new  areas.  Most  moss  plants  are 
perennials,  and  they  produce  both  oospores  and  asexual  spores 
on  new  branches  for  several  years. 

Summary.  Liverworts  and  mosses  constitute  a  group  of 
rather  simple  land  and  amphibious  plants.  They  are  of  par- 
ticular interest  because  they  show  some  of  the  steps  by  which 
simple  water  plants  become  adjusted  to  conditions  on  land. 
This  group  represents  the  most  complex  of  the  land  plants 
that  lack  a  conductive  system.  In  the  next  group,  the  ferns, 
the  plant  has  a  well-developed  water-conducting  and  food- 
conducting  system,  and  correlated  with  this  it  has  greater 
size  and  more  complex  tissue  systems. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Twenty-one 

1.  Make  a  field  trip  to  study  ferns  and  their  allies.     Ferns  are 
common  in  rich  woods  and  on  moist  cliffs.     Equise turns  may  be 
found  along  streams  and  railway  embankments,  and  in  swamps. 
The  club  mosses  occur  in  low  grounds,  in  rich  woods,  and  in  moist, 
sandy  depressions  along  lake  and  sea  beaches.     Tropical  forms 
may  be  seen  in  conservatories  and  greenhouses.     Note  especially 
the  relative  development  of  leaves,  stems,  and  roots.     Compare 
them  with  the  seed  plants  growing  in  the  same  habitats.     Have 
they  any  advantages  over  the  mosses,  liverworts,  and  algae  that 
live  near  them  ? 

2.  Examine  growing  plants  and  note  how  fern  leaves  unfold. 
Note  the  sporangia  and  spores  on  the  under  sides  of  the  leaves. 
Plant  some  of  the  spores  on  moist  soil  in  a  flower  pot  and  cover  with 
a  glass  plate.     Examine  the  prothalli  that  develop  from  the  spores. 

3.  If  field  work  is  impossible,  examine  a  collection  of  pressed 
specimens  of  ferns  and  allied  plants,  in  order  to  get  an  idea  of  the 
great  variety  of  forms  belonging  to  these  groups. 


282 


CHAPTER  TWENTY-ONE 

THE   FERNS   AND   THEIR   ALLIES 


THE  ferns  and  their  allies  are  a  large  assem- 
blage of  plants.  Like  the  simple  plants  that  we 
have  studied,  they  reproduce  by  spores  instead 
of  seeds ;  but,  like  the  seed  plants,  they  possess 
water-conducting  and  food-conducting  tissues. 
They  appeared  on  the  earth  earlier  than  the 
seed  plants,  and  were  more  abundant  during  the 
carboniferous  or  coal-making  period  of  the 
earth's  history  than  they  are  now.  Indeed, 
judging  by  the  fossils  in  coal,  ferns  and  fernlike 
plants  dominated  the  vegetation  of  that  time, 
and  the  seed  plants  were  much  less  prominent 
than  now.  Ever  since  that  period  the  seed 
plants  have  been  gaining  in  importance;  and 
now  the  ferns  have  been  to  a  large  extent 
displaced,  especially  in  the  temperate  and 
colder  parts  of  the  earth.  Three  groups  of 

these  plants  that  are  sufficiently  distinct  to  be 

*  *  .     FIG.  166.  The  walk- 

readily   recognized  occur  rather  commonly  in  ingfern.   New  plants 

North  America.     They  are  the  ferns,  equisetums,  are  developed  from 
and  club  moSSes. 

283 


284 


Science  of  Plant  Life 


FIG.  167.    A  tree  fern. 


The  Ferns  and  Their  Allies 


285 


FIG.  168.     A  large  tropical  fern  (Marratia),  with  leaves  over  30  feet  in  length. 
Philippine  Islands. 

The  ferns.  The  ferns  are  noted  for  the  large  size  of  their 
foliage  leaves.  In  temperate  regions  these  leaves  are  much 
branched  and  in  some  species  attain  lengths  of  from  3  to  6 
feet.  They  usually  rise  from  underground  stems,  which  may 
be  long,  slender,  and  horizontal  or  short,  thick,  and  upright. 
The  venation  of  the  leaves  differs  from  that  of  seed  plants 
in  being  forked ;  that  is,  each  vein  divides  into  two  smaller 
ones,  and  each  of  these  subdivides  in  the  same  manner.  The 
root  systems  of  the  ferns  are  all  small  and  scantily  branched  as 
compared  with  those  of  the  seed  plants.  This  is  probably 
the  reason  why  ferns  as  a  rule  are  confined  to  moist  habitats. 

In  the  moist  tropics  there  are  many  species  of  tree  ferns 
with  upright  columnar  stems  bearing  a  rosette  of  leaves  at 
the  summit.  The  stems  may  attain  heights  of  from  10  to 
50  feet,  and  they  bear  leaves  5  to  15  feet  in  length. 

The  equisetums,  or  horsetails.  In  swamps,  on  the  banks 
of  streams,  and  on  railroad  embankments  one  can  find  slender, 


286 


Science  of  Plant  Life 


columnar  plants  with  scale  leaves  and  spore-bearing  cones  on 
the  ends  of  the  upright  branches.  These  are  the  remnants  of 
a  group  that  made  up  a  large  part  of  the  vegetation  during 
the  carboniferous  period.  Many  of  the  species  are  evergreen. 

The  equisetums  are  for  the 
most  part  stream-margin  and 
swamp  plants,  and,  as  might  be 
expected,  the  stems  have  air 
cavities  extending  throughout. 
In  some  species  the  stems  de- 
velop at  each  node  whorls  of 
slender  branches,  which  to- 
gether form  a  brush,  suggesting 
the  common  name  "  horse-tail " 
for  the  group. 

Like  the  ferns,  most  of  the 
species  develop  an  extensive 
system  of  underground  stems 
from  which  the  upright  branches 
rise.  In  the  absence  of  foliage 
leaves,  the  stems  do  all  the 
photosynthetic  work.  The 
roots  are  small,  like  those  of 
the  ferns.  Spores  are  pro- 
duced in  the  cones  which  termi- 
nate the  upright  stems. 
The  club  mosses.  These 

FIG.  169.  The  common  field  equisetum.  are  creeping  and  trailing  ever- 
Rootstock  with  sterile  branches  (#),  green  plants  found  in  rich  woods 
^t±s^t^tr^  "^  bogs,  particularly  in  the 


pendages. 


northern  United  States.     They 


The  Ferns  and  Their  Allies  287 

are  much  used  for  decorative  purposes  at  Christmas  time 
and  are  sold  under  various  names  such  as  "  ground  pine  " 


FIG.  1 70.    Two  species  of  club  mosses  (Lycopodium) .    In  one  species  (left)  the  sporangia 
are  borne  in  the  axils  of  the  upper  leaves,  in  the  other  (right)  they  are  borne  in  terminal 


and  "  running  cypress."  The  main  stems  are  prostrate  on 
the  soil  and  may  be  many  feet  long.  They  are  anchored  at 
intervals  by  short  roots.  Numerous  upright  branches  rise 


288  Science  of  Plant  Life 

from  the  prostrate  stems.  The  stems  bear  large  numbers  of 
scale  leaves,  and  the  spores  are  produced  in  terminal  cones. 
The  spores  are  sold  in  drug  stores  under  the  name  "  lyco- 
podium  powder."  They  are  used  as  a  drying  powder  and 
in  the  manufacture  of  fireworks. 

The  club  mosses  were  of  large  size  and  very  abundant 
during  the  time  when  the  carboniferous  rocks  were  deposited, 
and  with  the  ferns  and  equisetums  they  formed  the  chief  part 
of  the  forests  of  that  geological  period.  The  coal  which  we 
now  burn  is  the  last  remnant  of  the  carbon  compounds  formed 
by  plants  from  the  carbon  dioxid  then  present  in  the  atmos- 
phere. Certain  of  the  club  mosses,  like  some  of  the  ferns, 
show  stages  in  the  transition  from  reproduction  by  spores 
to  reproduction  by  seeds.  The  club  mosses  and  equisetums 
have  highly  branched  stems  and  many  small,  scalelike  leaves. 
In  the  ferns  there  is  little  branching  of  the  stem  and  the  leaves 
are  large  and  often  much  divided. 

The  two  generations  of  the  ferns.  One  of  the  most  inter- 
esting features  of  the  life  history  of  the  fernlike  plants  is  the 
fact  that  there  are  two  very  different  plants  produced  during 
one  life  cycle.  Asexual  spores  are  produced  on  the  under 
surfaces  of  fern  leaves,  and  when  these  are  planted  on  soil 
and  allowed  to  germinate,  we  find  that  in  about  6  weeks  the 
soil  is  covered  with  small,  heart-shaped  thalli  which  look  very 
much  like  liverworts.  These  thalli  represent  one  generation 
of  the  fern  plant.  Each  thallus  consists  of  an  irregular  plate 
of  cells,  a  single  cell  layer  in  thickness  except  toward  the 
middle,  where  it  may  consist  of  several  layers  (Fig.  172). 
The  prothallus,  as  the  plant  is  called  in  this  stage,  is  anchored 
to  the  soil  by  one-celled  rhizoids.  It  manufactures  its  own 
food  and  has  an  existence  similar  to  that  of  a  liverwort. 


The  Ferns  and  Their  Allies 


289 


When  mature,  the  pro- 
thallus  produces  egg  cells 
and  sperms  in  small  organs 
on  its  under  surface.  The 
sperms  are  small  and  swim 
actively,  and  they  reach  the 
egg  cells  when  the  thallus 
is  wet.  The  sperm  unites 
with  the  egg  and  forms  an 
oospore.  The  formation  of 
the  oospore  completes  the 
life  of  this,  the  sexual,  gen- 
eration of  the  fern. 

The  oospore  germinates 
wrhile  still  attached  to  the 
little  prothallus,  and  from 
it  develops  the  large  fern 
plant  with  which  we  are 
familiar.  This  second  gen- 
eration has  true  roots, 
stems,  and  leaves,  and  on 
the  lower  side  of  the  leaves 
great  numbers  of  sporangia 
are  produced.  These  lie  in 
groups  which  may  be  seen 
with  the  naked  eye  as 
brown  dots.  Within  the  sporangia  asexual  spores  are  formed, 
and  because  asexual  spores  are  produced  by  this  generation, 
it  may  be  called  the  asexual  generation  of  the  plant. 

The  ferns,  therefore,  do  not  reproduce  themselves  directly. 
The  plants  with  the  large,  feathery  leaves  that  we  know  as 


FIG.  171.     Underground  stem,  roots,  and 
leaves  of  fern. 


290 


Science  of  Plant  Life 


ferns  produce  asexual  spores  from  which  small,  flat  prothalli 
develop.  The  prothalli  in  turn  produce  gametes  and  sexual 

spores,  which  then  germinate 
to  form  the  fern  plants  proper. 
This  alternation  of  two  unlike 
generations  in  the  life  history 
of  the  fern  is  most  interesting. 
The  alternation  of  an  asexual 
generation  with  a  small  sexual 
generation  occurs  in  mosses 
(page  279)  as  well  as  in  ferns, 
and  it  is  found  also  in  most 
of  the  other  great  plant 
groups.  However,  the  alter- 
nation of  generations  is  not 

FIG.  172.     Under  side  of  fern  prothallus,  , 

showing  egg-producing  organs  (archegonia)  SO    easy    to    Study    in    groups, 

(A),  the  sperm-producing  organs  (anther-  other  than  the  ferns,  Ul  which 

idia)  OB),  and  the  rhizoids  (C).  ,                                                   , 

the  two   generations  do  not 

grow  independently  of  each  other.  In  some  of  the  ferns  and 
also  in  some  of  the  club  mosses,  the  sporangia  and  the  sexual 
generations  show  stages  in  the  development  of  these  struc- 
tures into  the  seeds  of  the  higher  plants ;  but  the  origin  of 
seeds  is  too  complex  a  question  to  be  discussed  here. 

The  significance  of  the  conductive  system  in  plants.  The 
algae,  being  immersed  in  water,  have  a  constant  water  supply. 
The  fungi  have  an  adequate  water  supply  because  they  live 
inside  other  plants  that  have  water-conducting  tissue,  or 
because  they  grow  in  moist  soil  or  within  stumps  and  logs. 
A  water  balance  is  maintained  among  the  mosses  and  liver- 
worts by  their  growing  in  spongelike  masses  and  in  close 
contact  with  the  soil  water ;  but  within  these  plants  water 


The  Ferns  and  Their  Allies 


291 


can  be  transmitted  from  one  part  to  another  only  slowly  by 
diffusion  from  cell  to  cell.     They  have  no  water-conducting 


H 


FIG.  173.  The  life  history  of  a  fern.  The  prothallus  (A)  produces  egg  cells  and  sperms 
in  organs  on  the  lower  surface.  One  of  the  sperms  set  free  from  B  unites  with  an  egg 
cell  (shown  in  C),  and  produces  a  sexual  spore.  This  germinates  and  produces  the  leafy 
fern  plant  (D),  which  in  turn  produces  asexual  spores  in  sporangia  (F  and  G)  on  the 
lower  side  of  the  leaves.  By  the  bursting  of  the  walls  of  the  sporangium  (H),  the 
spores  are  set  free.  They  then  germinate  on  the  soil  (in  some  species  on  rocks  or 
trees)  and  produce  a  new  generation  of  prothalli  like  the  one  shown  in  A .  The  pro- 
thallus is  here  shown  about  4  times  its  natural  size. 

or  food-conducting  tissues,  and  for  this  reason  they  are  all  of 
small  size. 

The  development  of  a  conductive  system  is,  therefore,  an 
additional  step  in  the  complete  adjustment  of  plants  to  a  land 
environment.  With  a  conductive  system,  the  water  may  be 
carried  rapidly  from  one  part  of  a  plant  to  another.  Con- 
sequently stems  and  leaves  may  be  raised  far  above  the  ground 
level  and  yet  receive  sufficient  water  from  the  roots  to  replace 
that  lost  through  transpiration.  Likewise  an  adequate  supply 
of  food  may  be  transferred  to  the  roots,  which  makes  it  pos- 
sible for  them  to  live  in  the  soil,  where  they  are  unable  to 


292  Science  of  Plant  Life 

manufacture  food  for  themselves.  This  permits  a  great  in- 
crease in  the  size  of  the  plant  body,  and  makes  possible  a 
high  degree  of  specialization  in  plant  organs  and  tissues. 
As  far  as  is  known,  the  ferns  and  their  relatives  were  the 
first  plants  to  develop  a  conductive  system.  We  do  not 
know  how  the  conductive  system  originated,  but  its  coming 
into  existence  marked  perhaps  the  most  important  step  in 
the  progress  of  the  plant  kingdom. 

Summary.  The  ferns  and  their  allies  constitute  the  rem- 
nant of  a  very  ancient  and  important  group  of  spore  plants. 
In  the  study  of  the  evolution  of  seed  plants  they  are  of  in- 
terest (i)  because  they  have  two  entirely  separate  generations, 
a  small,  sexual  one  which  produces  gametes,  and  the  large, 
leafy  plant  which  produces  asexual  spores ;  (2)  because  they 
show  some  of  the  stages  in  the  transition  from  plants  which 
reproduce  only  by  spores  to  plants  that  produce  seeds ;  and 
(3)  because  they  were  the  first  plants  to  develop  a  conductive 
system.  The  presence  of  a  conductive  system  enables  these 
plants  to  raise  leaves  and  stems  well  above  the  soil  and  to 
expose  their  photosynthetic  tissues  to  the  light  far  better 
than  any  of  the  preceding  groups.  The  ferns  do  not  have 
extensive  root  systems,  and  consequently  are  more  closely 
confined  to  moist  habitats  than  are  the  seed  plants. 


Suggestions  for  Laboratory  and  Field  Work  to  Precede 
Chapter  Twenty-two 

1.  Field  work  or  a  trip  to  a  greenhouse  will  help  to  make  clear 
some  of  the  characteristics  of  seed  plants,  and  the  differences  in 
the  plants  belonging  to  the  several  divisions  of  the  group. 

2.  Examine  vegetative  shoots  of  pine,  hemlock,  spruce,, cypress, 
arbor  vitae,  or  other  available  conifers,  and  compare  their  leaves 
and  stems  with  those  of  the  ferns  and  angiosperms. 

3.  Material  collected  from  one  of  the  conifers   in  the  spring, 
showing  the  staminate  and  carpellate  cones,  should  be  studied  in 
the  laboratory.     Mature  cones  may  be  obtained  at  any  time,  and 
they  will  show  the  relation  of  seeds  to  scales. 


293 


CHAPTER   TWENTY-TWO 

SEED  PLANTS:   GYMNOSPERMS 

SEED  plants  form  the  most  conspicuous  part  of  the  earth's 
vegetation,  and  they  include  the  majority  of  the  plants  that 
are  of  interest  as  sources  of  food,  lumber,  and  fibers.  They 
are  the  plants  with  which  we  are  most  familiar,  and  for  this 
reason  the  first  seventeen  chapters  of  this  book  have  been  de- 
voted to  a  description  of  their  structures,  processes,  and  en- 
vironmental relations.  Here  the  great  groups  into  which  the 
seed  plants  are  divided  will  be  described,  and  some  of  the 
more  important  families  in  these  larger  groups  will  be  briefly 
discussed.  The  two  most  important  groups  of  living  seed 
plants  are  the  Gymno sperms  and  the  Anglos  perms. 

The  gymnosperms.  The  familiar  representatives  of  the 
gymnosperm  group  are  the  conifers,  or  cone-bearing  trees. 
The  name  "  gymnosperm "  (Greek :  gymnos,  naked,  and 
sperm,  seed)  suggests  the  most  distinctive  feature  of  the 
group.  The  seeds  are  borne  exposed  on  the  upper  surface  of 
scales,  which  make  up  the  cones.  They  are  not  inclosed  by 
a  pistil  wall  as  are  the  seeds  of  the  next  group. 

The  conifers.  The  conifers  have  scale  and  needle  leaves. 
The  red  cedar  and  arbor  vitae  are  of  the  scale-leaf  type ;  the 
pines,  spruces,  and  hemlocks  are  of  the  needle-leaf  type.  The 
stems  are  woody,  much  branched,  and  of  large  size.  Indeed, 
the  largest  trees  in  the  world,  the  giant  sequoias,  or  Big  Trees, 
of  California,  belong  to  this  group.  The  distinctive  feature 
of  gymnosperm  wood  is  the  absence  of  water-conducting 
tubes.  The  wood  cells  perform  the  double  function  of 
supporting  the  tree  and  conducting  the  water.  Many  of 
the  conifers  have  large  resin  tubes  extending  throughout 
the  plant.  Just  what  advantage  comes  to  a  plant  through 

294 


Seed  Plants :    Gymnosperms 


295 


FIG.  174.     Pine  forest,  with  an  occasional  maple,  birch,  or  aspen. 


296  Science  of  Plant  Life 

the  formation  of  resin  is  not  known.  Resin  is  a  valuable 
commercial  product,  and  in  the  Southern  states  the  long- 
leaf  pine  furnishes  the  crude  rosin  and  the  turpentine  of 
commerce. 

Leaves  of  conifers.  The  leaves  of  most  conifers  are  ever- 
green and  remain  on  the  trees  for  a  period  of  from  2  to  10 
years,  depending  somewhat  upon  climatic  conditions.  In 
general  they  last  longer  in  moist  habitats.  The  Northern 
larch  or  tamarack  and  the  Southern  cypress  are  deciduous 
trees  with  soft  needle  leaves  that  contrast  strongly  with  the 
hard  needle  leaves  of  the  evergreens ;  the  hardness  of  the 
evergreen  needles  makes  it  possible  for  them  to  withstand  the 
winter  droughts. 

Roots  of  conifers.  The  roots  of  conifers  are  far  better 
developed  than  are  those  of  the  ferns.  Like  deciduous  trees, 
they  have  root  systems  that  gradually  taper  from  the  stem 
base  and  spread  over  a  wide  area.  The  great  root  system 
makes  possible  the  development  of  a  large  crown  and  the 
exposure  of  a  greater  leaf  surface  than  in  the  ferns. 

Production  of  seeds.  The  production  of  seeds  in  conifers 
may  be  illustrated  by  the  pine.  Two  kinds  of  cones  are  pro- 
duced :  the  stamina te  cones  that  produce  the  pollen,  and  the 
ovulate  cones  that  bear  the  seeds  (Fig.  175).  The  staminate 
cones  are  short-lived  structures  of  early  spring.  Each  scale 
bears  two  pollen  sacs,  which  contain  the  pollen  grains.  The 
pollen  is  blown  about  by  the  wind,  and  each  grain  has  on  its 
sides  two  little  air  sacs  that  cause  it  to  be  carried  through 
the  air  easily.  It  is  produced  in  enormous  quantities ;  and 
when  it  is  shed,  it  oftentimes  colors  the  ground  yellow  in  the 
vicinity  of  pine  woods.  The  staminate  cone  of  the  conifers 
corresponds  to  a  staminate  flower  in  the  flowering  plants ; 


Seed  Plants :    Gymnosperms 


297 


it  lacks  a  calyx  and  a  corolla,  but  its  scales  produce  pollen  and 
therefore  correspond  to  the  stamens  of  a  lily  or  rose. 

Each  scale  of  the 
ovulate  cone  has  two 
ovules,  or  young 
seeds,  on  its  upper 
surface.  At  the  time 
when  the  pollen  is 
shed  these  scales 
stand  open,  or  are 
separated  from  one 
another,  so  that  the 
pollen  falls  on  them 
and  slides  down  to 
the  bases  of  the 
scales,  where  it  comes 
into  contact  with  the 
young  ovules.  Then 
the  scales  of  the  cone 

close  Up  tightly,  and    FlG-  L75-     Spray  of  Austrian  pine.     At  the  left  (above) 
i  u  .        j         is  a  i -year-old  and  (below)  a  2 -year-old  ovulate  cone. 

the  pollen  grains  de-   On  the  right  is  a  cluster  of  staminate  cones. 
velop   pollen   tubes. 

These  grow  down  into  the  ovules  and  produce  sperms  which 
finally  fertilize  the  eggs.  As  a  result  of  fertilization,  an 
embryo  is  produced,  and  the  ovule  walls  become  the  seed 
coats.  The  growth  of  the  pollen  tube  is  very  slow  in  coni- 
fers, and  it  is  only  at  the  close  of  one  or  two  years'  growth 
that  the  seeds  mature  and  the  ovulate  cones  die.  Then  the 
drying  out  and  spreading  of  the  scales  permits  the  winged 
seeds  to  fall  out  or  to  be  blown  out  and  carried  to  the  ground. 
In  many  cases  the  seeds  of  conifers  have  hard  waterproof 


298  Science  of  Plant  Life 

coats  which  may  cause  them  to  remain  dormant  for  long 
periods.  When  planted,  some  of  the  seeds  germinate  the 
first  season,  others  the  second,  and  some  not  until  the  third 
or  fourth  year.  This  tends  to  insure  some  of  the  seedlings 
favorable  conditions  for  growth.  All  the  seeds  will  germinate 
the  first  season  if  the  waterproof  outer  coat  is  ground  off  by 
shaking  the  seeds  with  sharp  sand.  In  tree  nurseries  seeds 
are  usually  treated  in  this  way,  so  as  to  insure  the  rapid  and 
uniform  development  of  the  seedlings. 

The  conifer  forests  of  North  America.  The  greater  part 
of  the  forested  area  of  this  continent  is  occupied  by  conifers. 
One  of  the  striking  characteristics  of  conifer  forests  is  the  fact 
that  extensive  areas  are  often  occupied  by  a  single  species. 
In  the  Northern  Evergreen  Forest,  which  stretches  from  the 
lower  St.  Lawrence  basin  to  Alaska  and  extends  southward  in 
the  Alleghanies  to  northern  Alabama,  the  principal  trees  are 
the  white  and  black  spruce,  white  pine,  hemlock,  jack  pine, 
balsam  fir,  tamarack,  and  arbor  vitae.  The  spruces  are  the 
principal  source  of  the  wood  pulp  used  in  the  manufacture  of 
paper.  The  white  pine  formerly  furnished  the  most  durable 
and  most  desirable  lumber  in  the  Eastern  states,  but  the 
supply  of  this  lumber  is  now  almost  exhausted. 

In  the  Southeastern  Evergreen  Forest,  which  extends  along 
the  coastal  plain  from  New  Jersey  to  Texas,  the  long-leafed 
and  short-leafed  pines,  the  loblolly  pine,  the  cypress,  and  the 
red  cedar  are  the  most  important  timber  trees.  The  long- 
leafed  pine  is  the  source  of  rosin  and  turpentine,  and  with 
other  pines  and  the  cypress  it  supplies  much  of  the  lumber 
for  interior  finishings  and  other  building  purposes.  Cypress 
lumber  is  noted  for  its  durability  in  moist  situations  where 
other  lumber  soon  decays  through  the  attacks  of  fungi. 


Seed  Plants :    Gymnosperms 


299 


FIG.  176.     Map  showing  the  distribution  of  the  forests  of  North  America. 

The  Western  Evergreen  Forest  covers  the  Rockies,  Sel- 
kirks,  and  Sierra  Nevada  and  Coast  ranges,  from  southern 
Alaska  to  Mexico.  Throughout  this  vast  area  the  dominant 


300  Science  of  Plant  Life 

trees  are  the  Western  yellow  pine,  Douglas  fir,  lodgepole  pine, 
redwood,  pinon,  and  Sitka  spruce;  and  there  are  numerous 
less  important  cedars,  spruces,  firs,  and  pines.  With  the  in- 
creasing demand  for  lumber  and  the  gradual  destruction  of 
the  Eastern  forests,  more  and  more  of  the  wood  from  the 
Western  forests  is  reaching  Eastern  lumber  markets.  The 
seeds  of  the  pinon  are  large  and  pleasantly  flavored,  and 
formerly  they  furnished  much  of  the  food  for  the  Indians  of 
California  and  the  lower  Great  Basin  region. 

The  significance  of  reproduction  by  seeds.  In  a  former 
chapter  (page  207)  some  details  of  the  manner  in  which  a  seed 
is  produced  were  given,  and  it  was  pointed  out  that  the  seed 
is  a  structure  containing  a  young  plant  (embryo)  developed 
from  a  fertilized  egg.  The  seed  is  better  fitted  to  undergo 
a  period  of  dormancy  and  to  carry  a  plant  over  an  unfavor- 
able period  than  are  the  small  spores  of  ferns,  mosses,  and 
fungi,  or  even  the  resting  spores  that  are  produced  by  certain 
algal  forms.  This  is  because  the  seed  contains  (i)  a  plant 
already  partly  developed,  (2)  a  larger  supply  of  nourish- 
ment to  start  the  young  plant  when  development  begins,  and 
(3)  seed  coats  that  afford  better  protection  during  the  dor- 
mant period.  So  efficient,  indeed,  is  the  seed  in  passing  over 
periods  unfavorable  for  growth,  that  nearly  all  seeds  will 
retain  their  vitality  for  from  one  to  several  years ;  and  the  seeds 
of  certain  plants  have  been  found  to  be  alive  after  25  years. 

Summary  of  the  development  of  the  plant  kingdom.  At 
this  point  it  may  be  well  to  review  the  most  notable  steps  in 
the  development  of  plant  life  on  the  earth.  These  steps  are : 

(i)  The  development  from  previously  existing  water 
plants  of  land  forms  with  epidermis,  rhizoids,  and  other  ad- 
justments to  a  land  environment. 


Seed  Plants  :    Gymnosperms  301 

(2)  The  development  of  a  conductive  system  which  made 
possible  the  growth  of  aerial  parts  situated  at  a  distance 
from  the  water  supply,   and  which  also  made  possible  the 
transportation  of  food  to  roots  that  live  in  the  dark  and  there- 
fore are  not  able  to  make  their  own  food. 

(3)  The  development  of  seeds  as  a  means  of  dispersing  the 
plant  and  of  preserving  it  during  seasons  unfavorable  for 
vegetative  life. 

(4)  The  development  of  an  extensive  root  system  by  which 
larger  amounts  of  water  and  mineral  salts  are  made  acces- 
sible.    Correlated   with   this   is   the   development   of   large, 
much-branched  tree  forms. 

(5)  The. development  of  true  flowers  made  up  of  floral  leaves, 
stamens,  and  pistils.     With  this  step  came  insect  pollination 
and  the  production  of  seeds  inclosed  in  an  ovulary.     In  some 
cases  the  ovulary  is  fleshy  and  at  maturity  becomes  an  edible 
fruit. 


Suggestions  for  Laboratory  Work  to  Precede  Chapter 
Twenty-three 

1.  A  lily,  tulip,  or  wild  onion  may  be  used  in  studying  the 
flowers  of  the  monocotyledons.    Attention  should  also  be  given 
to  the  leaf  and  stem  characters  of  this  group.      Narcissus  bulbs 
grown  in  water  will  furnish  excellent  material. 

2.  Any  convenient  dicotyledon  may  be  used  to  show  the  type 
of  flower  and  the  associated  leaf  and  stem  characteristics.     Ge- 
raniums, oxalis,  and  impatiens  may  be  obtained  at  all  seasons  of 
the  year.     In  autumn  and  spring  many  plants  are  available. 

3.  A  collection  of  the  more  important  grain  plants,  fibers,  and 
fiber  plants  is  easily  made.     These  will  be  helpful  in  studying  the 
economic  uses  of  plants. 

4.  The  examination  of  pressed  specimens  representing  the  more 
important  families  of  angiosperms  will  be  an  aid  in  learning  family 
characteristics.     Fresh  specimens  are  always  to  be  preferred,  if 
they  can  be  obtained. 


302 


CHAPTER   TWENTY-THREE 

SEED  PLANTS:   ANGIOSPERMS 


FIG.  177.    Roadside  clump  of  bamboo,  Philippine  Islands.    The  bamboo  is  a  very 
important  tropical  monocot. 

ANGIOSPERMS  (Greek :  angio,  receptacle,  and  sperm,  seed) 
form  the  second  division  of  seed  plants.  The  seeds  are  in- 
closed in  a  pistil  which  later  ripens  into  a  pod  or  fruit.  The 
Angiosperms  make  up  by  far  the  largest  part  of  the  present 
vegetation  of  the  earth.  There  are  more  than  130,000  species, 
as  contrasted  with  500  species  of  Gymnosperms  and  about 
4500  species  belonging  to  the  fern  group.  In  many  forms  the 
stamens  and  pistils  are  surrounded  by  brightly  colored  floral 
leaves.  As  compared  with  other  groups  of  seed  plants,  a 
most  striking  diversity  of  vegetative  and  reproductive  struc- 
tures is  shown  within  the  group.  This  diversity  of  form  en- 
ables Angiosperms  to  live  in  all  land  and  water  habitats,  from 
the  margin  of  the  ocean  to  alpine  summits,  and  from  the  tropics 
to  the  polar  deserts. 

303 


304  Science  of  Plant  Life 

Diversification  among  the  Angiosperms.  The  great  dif- 
ferences in  the  stems  of  Angiosperms  may  be  realized  by  call- 
ing to  mind  the  elm,  palm,  cactus,  dandelion,  grape,  morning- 
glory,  bamboo,  tumbleweed,  pondweed,  water  lily,  dodder, 
and  duckweed.  The  leaves  may  be  simple  or  divided  into 
leaflets ;  and  in  size  they  range  from  the  minute  scales  of  the 
heather  to  the  leaves  of  the  palm  and  banana,  which  may  be 
from  20  to  30  feet  long.  The  form,  color,  and  venation  of 
the  leaves  show  a  corresponding  diversity,  so  that  all  shapes 
and  sizes  of  leaves  may  be  found  within  the  group.  The 
Angiosperms  have  extensively  developed  root  systems  that 
are  capable  of  anchoring  the  plants  and  absorbing  water  under 
the  greatest  variety  of  soil  conditions.  The  reproductive 
structures  are  very  diverse  and  are  more  complex  than  in  the 
Gymnosperms.  In  addition  to  a  great  number  of  methods 
of  vegetative  multiplication  by  leaves,  stems,  and  roots,  there 
is  the  production  of  seeds  from  flowers.  In  the  higher 
Angiosperms  there  are  brightly  colored  floral  parts  which  are 
helpful  in  securing  the  transfer  of  pollen  by  insects.  The 
seeds  are  inclosed  in  fruits  which  protect  the  developing  ovules 
and  frequently  aid  in  the  scattering  of  the  seeds. 

Two  great  groups  of  Angiosperms.  The  Angiosperms 
naturally  fall  into  two  great  groups :  the  monocotyledons  and 
the  dicotyledons.  These  groups  have  already  been  discussed 
(page  209),  and  only  a  general  summary  of  their  character- 
istics will  be  presented  here. 

Monocotyledons.  The  Monocotyledons  have  their  floral 
parts  usually  in  groups  of  three  (rarely  in  four) ;  the  bundles 
are  closed  and  scattered  throughout  the  pith  of  the  stem; 
in  most  forms  the  veins  of  the  leaves  are  parallel ;  and  the 
embryo  has  but  one  well-developed  cotyledon.  With  the 


Seed  Plants :    Angiosperms 


305 


FIG.  178.    A  deciduous  forest  In  Illinois.    The  trees  are  oak,  hickory,  and  elm, 
important  and  typical  dicot  trees. 

exception  of  the  palms,  bamboos,  and  a  few  other  species,  the 
Monocotyledons  are  herbs  ;  and  the  tissues  of  even  the  woody 
Monocotyledons  are  generally  less  woody  than  are  those  of 
the  conifers  and  the  dicotyledonous  trees  and  shrubs.  This 
great  assemblage  of  flowering  plants  includes  not  fewer  than 
25,000  species. 

Dicotyledons.  The  floral  parts  of  Dicotyledons  are  usually 
in  groups  of  five  or  four ;  the  vascular  bundles  are  arranged 
in  a  circle  about  a  central  pith ;  the  bundles  are  open,  which 
makes  possible  an  increase  in  stem  diameter  and  permits  forms 
like  the  trees  to  reach  a  great  size ;  the  leaves,  with  few 
exceptions,  are  net-veined ;  and  the  embryo  has  two  coty- 
ledons. In  size  the  Dicotyledons  range  from  the  smallest 
herbs  to  the  largest  of  our  hardwood  trees. 


306 


Science  of  Plant  Life 


FIG.  179.     Coconut  palm  in  fruit.     The  palm  family  is  very  important  in 
the  tropics. 


Seed  Plants :    Angiosperms 


3°7 


FIG.  1 80.  Transplanting  lowland  rice  in  the  Philippines.  The  seed  is  germinated 
in  beds,  and  at  the  opening  of  the  rainy  season  the  seedlings  are  transplanted  to  the 
flooded  fields.  The  rice  plant  and  other  members  of  the  grass  family  furnish  most 
of  the  food  supply  of  animals  and  men. 

The    important    families    of    the    Monocotyledons.     The 

Monocotyledons  are  usually  grouped  in  about  forty  families, 
four  of  which  are  of  surpassing  interest.  These  four  families 
are  the  palms,  grasses,  lilies,  and  orchids. 

The  palm  family.  The  palms  form  a  large  group  abundant 
throughout  the  tropics.  They  resemble  the  tree  ferns  and 
cycads  in  having  columnar,  unbranched  stems.  The  stem 
increases  in  thickness  during  the  first  few  years  and  then  grows 
only  at  the  top.  The  leaves  are  often  branched  and  of  gigantic 
size.  In  the  fan  palms  they  are  rounded  and  split  radially. 

Palms  are  very  useful  to  the  natives  of  the  tropics,  furnish- 
ing much  of  the  material  for  thatching  roofs  and  for  the  weav- 
ing of  mats,  hats,  and  bags.  They  also  produce  fruits  and 
seeds,  like  the  date  and  coconut,  that  are  most  important 


3o8 


Science  of  Plant  Life 


U.  S.  Dept.  of  Agriculture 
FIG.  181.     Harvesting  rice  in  California. 

sources  of  food.  The  rattan  is  a  climbing  palm,  for  the 
"  fiber  "  of  which  there  is  a  world- wide  market.  This  fiber, 
which  consists  of  flat  or  cylindrical  strips  of  the  stem,  is  used 
in  the  making  of  chairs,  tables,  and  baskets. 

The  grass  family.  The  grasses  furnish  the  bulk  of  the  food  of 
animals  and  men;  and  with  the  possible  exception  of  the 
trees,  no  other  group  of  plants  compares  with  them  in  use- 
fulness. They  form  the  natural  covering  of  a  large  part  of  the 
earth's  surface.  The  prairies  and  plains  of  North  America, 
the  steppes  of  Russia,  and  the  pampas  of  Argentina  are  vast 
areas  that  were  originally  dominated  by  grasses.  These  sup- 
ported great  herds  of  grazing  animals  before  the  coming  of 
man,  and  since  that  time  they  have  been  the  centers  for  cattle, 
horse,  and  sheep  production.  Grasses  are  cultivated  on  most 
farms  to  furnish  pasturage  for  the  live  stock. 

The  stems  of  grasses  are  cylindrical  and  usually  hollow, 
except  at  the  nodes.  The  leaves  are  usually  two-ranked, 
the  basal  portion  forming  a  sheath  about  the  stem,  as  may  be 


Seed  Plants :    Angiosperms 


3°9 


seen  on  a  stalk  of  corn.  The  flowers  are  borne  on  spikelets 
surrounded  by  green  bracts. 

The  grasses  reach  their  greatest  size  in  the  bamboos.  These 
have  long  been  one  of  the  chief  sources  of  lumber  in  eastern 
Asia  and  the  East  Indies.  From  the  bamboos  come  the 
materials  for  the  native  houses,  furniture,  and  a  large  number 
of  articles  of  household  use.  In  subtropical  America  these 
plants  are  being  used  as  windbreaks  for  the  pineapple  fields 
and  citrus  orchards. 

Sugar  cane  and  sorghum  are  members  of  the  grass  family 
that  are  cultivated  for  their  sugar.  The  former  is  the  source 
of  the  sugar  that  is  produced  in  our  Gulf  Coast  region  and  in 
the  tropics.  Sorghum  is  grown  farther  north  and  is  used 
for  the  manufacture  of  molasses.  Broom  corn  is  closely  re- 
lated to  sorghum,  and  is  grown  for  the  stiff  branches  of  the 
flower  cluster,  which  are  used  in  the  manufacture  of  brooms 


Bureau  of  Agriculture,  P.  I. 

FIG.  182.     Japanese  cane,  a  near  relative  of  the  sugar  cane,  grown  as  a  fodder  crop 
in  the  Philippines. 


310  Science  of  Plant  Life 

and  brushes.  Kafir  corn  and  milo  are  other  relatives  of  the 
sorghum  that  have  become  important  forage  crops  in  parts 

of  our  country  that  are  too  dry 
for  the  successful  production  of 
corn. 

But  the  most  important  of  all 
the  grasses  are  the  grains  :  wheat, 
rye,  barley,  oats,  rice,  millet,  and 
corn.  Most  of  the  world's  food  is 
derived  from  these  plants ;  and, 
depending  upon  the  yield  of  these 
several  crops,  human  beings  are 
well  supplied  with  food  or  famine 
prevails.  All  other  foods  are  of 
secondary  importance  to  the 
grains. 

The  lily  family.  This  is  one  of 
the  important  families  of  the 
monocots  because  of  the  large 
number  of  species,  their  wide 
distribution,  and  the  beautiful 
flowers  that  are  borne  by  many 
of  them.  The  flowers  usually 
have  six  colored  parts  forming 
the  floral  envelope.  There  are 
FIG.  183.  The  wood  lily.  Many  sjx  stamens,  and  the  pistil  is  made 

members  of  the  lily  family  are  cul-  .,    .   .  r~^ 

tivated  for  their  flowers.  up  of  three  divisions.     The  onion 

and  asparagus  are  members  of  the 

lily  family  that  are  used  for  food.  The  Mexican  century 
plants  and  the  New  Zealand  flax  are  fiber-producing  plants 
that  belong  to  this  group.  Many  of  the  bulbous  forms  like 


Seed  Plants :    Angiosperms 


the  Easter  lily,  tulip,  hyacinth,  and  narcissus  are  of  com- 
mercial importance  because  of  their  flowers. 

The  orchid  family.  This  is 
a  large  family  most  abundant 
in  the  tropics,  and  it  includes 
about  a  fifth  of  all  the  mono- 
cots.  In  temperate  America 
there  are  many  species  grow- 
ing in  bogs,  swamps,  and 
shaded  woods,  some  of  which 
are  noted  for  their  beautiful 
flowers.  The  pink  moccasin 
flower  and  the  yellow  lady's 
slipper  are  examples.  In  the 
tropics  most  of  the  species 
found  are  epiphytes  (page 
189).  These  species  are 
world-renowned  for  their 
marvelous  shapes  and  for 
the  coloring  of  their  flowers. 
Among  flower  fanciers  the  FlG-  l84-  A  tr°Pical  epiphytic  orchid. 

..  .  The  orchids  are  noted  for  thefr  varied  and 

orchids  are  the  most  sought-  beautifui  flowers. 

after   plants,    and   prices    of 

more  than  $1000  have  been  paid  for  single  plants  of  the  rarer 

species.     They  hybridize  freely,  and  there  are  innumerable 

hybrid  varieties  in  cultivation. 

Important  families  of  the  dicotyledons.  This  division  of 
the  flowering  plants  contains  over  200  families  and  more  than 
100,000  species.  Consequently  only  a  few  conspicuous  fami- 
lies can  be  discussed  here,  and  they  will  be  dealt  with  briefly. 
In  this  group  belong  our  broad-leafed  trees  and  shrubs,  and 


3I2 


Science  of  Plant  Life 


the  vast  majority  of  our  herbs.  The  poplars,  oaks,  hickories, 
maples,  and  elms,  and  the  chestnut,  beech,  and  ash,  largely 
make  up  the  Deciduous  Forest  of  the  eastern  United  States, 
which  dominates  the  area  east  of  the  Great  Plains  between  the 
upper  Lake  region  and  the  Gulf  coastal  plain. 

The  mustard  family.  This  family  includes  the  mustards, 
cabbages,  radishes,  turnips,  and  cresses.  It  is  of  great 
economic  importance.  The  flower  is  characterized  by  four 
petals  placed  at  right  angles  in  the  form  of  a  cross.  The 
development  of  cultivated  plants  from  wild  species  is  nowhere 
better  exemplified  than  in  the  case  of  the  cabbage.  The  small 

wild  plant  from  which  it  was 
derived  produced  neither 
heads  nor  swollen  stems. 
From  it,  by  the  selection  of 
mutations,  have  been  devel- 
oped several  distinct  forms  of 
cabbage  and  also  cauliflower, 
Brussels  sprouts,  kohl-rabi, 
kale,  and  collards.  These 
differ  so  much  in  appearance 
that  one  would  scarcely  guess 
that  they  had  a  common  wild 
origin.  The  histories  of  cul- 
tivated plants  afford  many  ex- 
amples of  far-reaching  changes 
in  the  forms  of  plants,  brought 
about  in  the  same  way. 
The  rose  family.  This 

FIG.  185.    Wild  rose,  and  a  vertical  section    famjly     js      characterized      by 

of  the  flower.     Most  of  the  important  tree  . 

and  shrub  fruits  belong  to  the  rose  family,    flowers    With    tlVC   petals,    tlVC 


Seed  Plants:    Angiosperms 


sepals,  and  many  stamens.  The  fruits  are  fleshy,  and  in 
many  species  they  are  edible.  The  group  is  of  particular 
interest  to  the  horticulturist, 
as  it  includes  most  of  our  im- 
portant tree  and  shrub  fruits, 
as  well  as  many  beautiful 
wild  and  cultivated  flowering 
plants.  Among  the  fruits  in- 
cluded are  strawberries,  rasp- 
berries, dewberries,  and  black- 
berries; apples,  pears,  and 
quinces ;  and  peaches,  plums, 
cherries,  prunes,  and  almonds. 
All  these  cultivated  fruits,  as 
compared  with  their  wild  an- 
cestors, have  been  improved 
in  flavor  and  greatly  increased 
in  size  and  productiveness. 

The    legume   family.      The 
legumes  received   their  name 

from  the  peculiar  fruit  or  pod  which  contains  the  seeds. 
Typical  legumes  are  the  pea  and  the  bean.  In  the  best-known 
members  of  the  family  the  flower  has  one  large  upper  petal 
which  stands  erect,  two  lateral  petals  forming  wings,  and  two 
lower  petals  united  and  inclosing  the  pistil  and  stamens. 
The  sweet  pea  shows  the  typical  flower  form. 

There  are  several  reasons  for  the  great  importance  of  the 
legume  family :  (i)  beans,  soy  beans,  and  peas  furnish  food 
material  that  contains  a  high  percentage  of  protein;  (2) 
all  legumes,  through  the  aid  of  bacteria  in  their  root  nodules, 
accumulate  nitrogen  compounds  (page  258) ;  (3)  clover, 


FIG.  1 86.     Sweet  pea,  and  a  vertical 
section  of  the  flower. 


Science  of  Plant  Life 


FIG.  187.  Peppermint.  The  square 
stems  and  opposite  leaves  are  char- 
acteristic of  the  mint  family. 


alfalfa,  and  soy  beans  supply  large 
quantities  of  rich  hay  for  horses, 
cattle,  and  sheep.  The  sweet  peas 
are  among  the  most  beautiful  of 
cultivated  flowering  plants.  The 
locust,  which  furnishes  a  very  en- 
during wood,  also  is  a  legume ;  and 
many  of  the  valuable  woods  of 
tropical  forests  belong  to  this  group. 
In  the  tropics  and  subtropics  one 
of  the  divisions  of  this  family  is  rep- 
resented by  the  acacias,  in  which 
the  flowers  are  radial  and  have 
numerous  long  stamens  that  give 
the  flower  clusters  the  appearance 
of  balls  of  coarse  hair.  The  acacias 
are  frequently  cultivated  as  or- 
namental trees,  and  they  are  a 
source  of  commercial  gums.  Re- 
lated to  the  acacias  are  the  sensi- 
tive plants  (Mimosa) ,  one  of  which 
is  shown  on  page  44.  In  South 
Africa  flat-topped  Mimosa  trees 
are  common  and  form  one  of  the 
characteristic  features  of  the  land- 


scape. 

The  mint  family.    This  family 
is  marked  by  square  stems,  peculiar 
two-lipped  flowers,  opposite  leaves, 
FIG.  188.    Climbing  nightshade,  an(^  jn  manv  species  an  aromatic 

one  of  the  wild  species  belonging  .  . 

to  the  potato  family.  odor.     Peppermint  and  spearmint 


Seed  Plants:    Angiosperms 


315 


are  now  cultivated  extensively  on 
bog  soils  in  the  northern  United 
States,  for  the  production  of  pep- 
permint oil  and  menthol.  Catnip, 
scarlet  sage,  water  horehound,  and 
hyssop  are  common  plants  belong- 
ing to  this  family. 

The  potato  family.  This  family  is 
composed  of  herbs  and  small  woody 
plants  with  expanded  or  tubular 
flowers.  The  fruits  are  capsules  or 
berries  containing  numerous  seeds. 
Among  the  useful  representatives 
are  the  potato  and  the  tomato, 
eggplant,  and  red  pepper.  Several 
plants  containing  narcotics,  of 
which  the  best-known  example  is 
tobacco,  belong  here.  Common 
weeds  that  belong  to  the  potato 
family  are  the  nightshade,  horse 

,  ,1  *      ,  i    T-  FIG.  189.    Wild  carrot.    The  flat- 

nettle,  ground  cherry,  and  Jimson  topped  flower  duster  characterizes 

Weed.  the    family    to    which    the    carrot 

The  carrot  family.     This  family  belongs' 

is  readily  recognized  by  the  flat-topped,  much-branched 
flower  cluster  (umbel).  The  parsnips,  carrots,  celery,  and 
sweet  cicely  are  familiar  examples.  In  addition  to  the  food- 
yielding  forms,  the  water  hemlocks  should  be  known  because 
of  their  poisonous  character. 

The  heath  family.  Small,  bell-shaped  flowers  and  leathery 
leaves  are  characteristics  of  the  heath  family.  To  it  belong 
the  cranberries,  blueberries,  huckleberries,  low-growing  tea- 


316 


Science  of  Plant  Life 


berry,    trailing   arbutus,   laurel,   azalea,   and   rhododendron. 
The  cranberry  has  long  been  cultivated  on  sandy  bog  lands. 

Blueberries  are  just  coming 
into  cultivation,  and  much 
may  be  expected  of  them  in 
the  future.  They  grow  best 
on  acid  soils,  and  are  capable 
of  great  improvement  in  size 
and  flavor  through  hybridiza- 
tion and  selection. 

The  composite  family.  The 
composite  family  is  the  cul- 
minating family  of  the  dicots, 
including  more  than  12,000 
species,  or  nearly  one  tenth 
of  all  seed  plants.  The  dis- 
tinguishing character  of  the 
family  is  that  the  small  flow- 
ers are  closely  grouped  in 
heads,  so  that  the  entire 
flower  clusters,  in  forms  like 
the  sunflowers,  asters,  chrys- 
anthemums, and  dandelions, 
are  often  mistaken  for  single  flowers.  The  calyx-like  outside 
covering  is  merely  several  rows  of  small  leaves  (bracts) .  The 
whole  inner  part  of  the  head  is  made  up  of  many  individual 
flowers,  each  with  its  own  stamens,  pistils,  and  corolla.  Some- 
times the  flowers  are  all  alike,  as  in  the  dandelion.  Sometimes 
the  outer  flowers  are  petaloid,  as  in  the  sunflower,  while  the 
inner  flowers  are  tubular  and  less  conspicuous.  The  floricul- 
turist has  modified  the  chrysanthemum,  aster,  and  dahlia  to 


FIG.  190. 


Mountain  laurel,  a  member  of 
the  heath  family. 


Seed  Plants :   Angiosperms 


317 


such  an  extent  that  there  are  now  hundreds  of  forms  in  culti- 
vation that  bear  little  resemblance  to  the  wild  stock  from 
which  they  were  derived. 


FIG.  IQI.     Flower  clusters  of  dahlia,  sunflower,  and  thistle,  members  of  the  composite 
family.     The  small  flowers  are  collected  in  heads,  which  are  characteristic  of  the  family. 

In  the  United  States  the  composites  bloom  chiefly  in  summer 
and  autumn,  and  often  conspicuously  color  the  landscape. 
The  daisies,  goldenrods,  and  asters  of  the  Eastern  states  and 
the  sunflowers  of  the  prairie  states  are  well  known.  The  rag- 
weeds, which  are  troublesome  as  weeds,  are  one  of  the  chief 
causes  of  hay  fever.  Their  pollen  is  highly  irritating  to  the 
mucous  membrane  of  the  nose,  and  it  is  advisable  that  city  and 
village  streets  be  kept  clear  of  them. 

Many  other  families  of  the  dicots  are  of  interest  and  im- 
portance, but  descriptions  of  them  must  be  sought  in  more 
specialized  botanical  texts. 


CHAPTER   TWENTY-FOUR 

THE  EVOLUTION   OF  PLANTS 

THOSE  who  have  studied  plants  most  have  been  led  to  the 
conclusion  that  simple  plants  lived  first  on  the  earth,  and 
that  from  these  simple  forms  all  the  varied  and  highly  com- 
plex plants  of  today  have  been  derived ;  that  is,  they  believe 
that  the  present-day  plants  were  evolved  from  those  that 
existed  on  the  earth  in  former  times.  Some  of  the  simple 
plants  of  the  past  still  persist,  and  many  plants  of  inter- 
mediate degrees  of  complexity  survive ;  but  during  the  de- 
velopment of  the  plant  world,  new  and  increasingly  complex 
forms  have  been  produced,  and  these  higher  forms  now 
dominate  the  vegetation  of  the  earth.  The  process  by  which 
the  plants  of  today  have  come  from  the  plants  of  the  past 
is  called  evolution  (Latin:  evolutio,  an  unrolling).  Evolu- 
tion, with  regard  to  plants,  implies  (i)  that  the  plants  of  to- 
day are  the  modified  descendants  of  earlier  forms,  (2)  that 
modifications  are  going  on  now  as  in  the  past,  and  (3)  that 
the  new  plants  of  the  future  will  be  evolved  from  plants  now 
living  through  modification  of  present  plant  forms. 

The  proofs  of  evolution  in  plants  have  been  gathered  from 
many  sources  by  many  different  students.  These  proofs 
include  the  evidence  furnished  (i)  by  plant  remains  found  in 
rocks  and  coal,  (2)  by  the  distribution  of  plants  on  the  earth's 
surface,  (3)  by  the  remarkable  similarity  of  organs,  tissues, 
and  cells  among  the  thousands  of  plants  now  in  existence, 
(4)  by  the  similarity  in  the  life  histories  of  all  plants,  (5)  by 
intergrading  species,  (6)  by  the  experience  of  plant  breeders 
and  the  history  of  our  cultivated  plants. 

The  record  of  plants  of  the  past.  When  a  leaf  falls  from  a 
tree  on  soft  mud,  it  may  become  imbedded  in  it.  Later  the 

318 


The  Evolution  of  Plants 


319 


mud  may  be  covered  by  other  layers  of  sediment.  When  the 
mud  dries,  a  perfect  imprint  of  the  outline  and  veins  may  be 
left.  As  time  goes  on  and 
the  mud  becomes  more 
deeply  buried,  it  may  harden 
into  rock  and  retain  the  im- 
print of  the  leaf  as  a  record 
of  a  plant  that  lived  when 
the  rock  was  merely  soft 
mud.  In  this  way  leaves, 
fruits,  seeds,  stems,  and 
roots  have  left  their  im- 
prints to  testify,  thousands 
and  millions  of  years  after- 
ward, to  their  former  ex- 
istence. 

Plant  remains  also  ac- 
cumulate in  deep  water  or 
in  water  containing  large 
amounts  of  mineral  matter  in  solution.  In  such  places  they 
may  not  decay,  but  the  material  of  which  they  are  composed 
may  gradually  be  oxidized  and  replaced  by  the  mineral 
substances  in  the  water.  Under  the  most  favorable  condi- 
tions the  internal  structures  of  the  plant  are  preserved.  As 
animal  remains  are  preserved  in  the  same  way,  we  have  in  the 
rocks  a  record  of  the  plants  and  animals  of  the  past.  These 
petrified  plant  and  animal  remains  and  the  plant  and  animal 
imprints  from  former  geological  ages  are  called  fossils. 

When  large  collections  of  fossils  are  studied,  we  find  in  the 
oldest  fossil-bearing  rocks  few,  if  any,  plant  remains.  This 
is  as  we  should  expect,  for  plants  like  algae  and  liverworts  are 


FIG.  192.     Fossil  imprints  of  fern  leaves  in 
a  rock  of  the  Carboniferous  period. 


320  Science  of  Plant  Life 

so  minute  and  delicate  that  we  cannot  hope  to  find  their  fossil 
remains.  Yet  we  know  that  plants  of  some  kind  did  exist 
on  the  earth  at  that  time,  for  the  fossils  of  corals  and  of  shell 
animals  are  abundant  in  these  rocks,  and  without  plants  to 
manufacture  food  for  them,  life  would  have  been  impossible 
for  these  animals.  In  rocks  that  were  formed  later  than 
these  first  fossil-bearing  strata,  remains  of  fernlike  plants  are 
found.  Later  there  were  seed-bearing  fernlike  plants  and 
several  groups  of  plants  intermediate  between  the  ferns  and 
modern  gymnosperms.  Still  later,  gymnosperms  resembling 
in  many  ways  our  coniferous  trees  became  prominent,  and 
then  came  Angiosperms  with  very  simple  flowers  of  the  wind- 
pollinated  type.  Finally,  Angiosperms  with  conspicuous 
insect-pollinated  flowers  appeared.  There  is  reason  to  be- 
lieve that  the  first  seed  plants  and  the  first  flowering  plants 
were  woody  shrubs  and  trees,  and  that  the  flowering  herbs 
came  last. 

The  meaning  of  the  record.  The  fossil  record  of  plants 
reaches  many  millions  of  years  into  the  past,  and  it  shows 
that  changes  in  plants  came  about  only  very  slowly.  The 
plants  of  each  geological  period  resembled  in  some  respects 
those  of  the  previous  period ;  in  each  period  certain  new 
characters  were  added  to  the  old  ones,  or  they  replaced  the 
old  ones.  To  reproduction  by  spores  was  added  reproduction 
by  seeds.  The  first  seeds  were  exposed  on  the  sides  of  foliage 
leaves ;  later  came  seeds  borne  on  scales  arranged  in  cones ; 
and  finally  came  seeds  inclosed  in  pistils. 

Leaves  and  stems  also  show  progressive  changes  in  form 
and  structure.  Although  the  fossil  record  is  very  fragmen- 
tary, being  made  up  of  chance  imprints  and  petrified  remains, 
it  is  possible  in  some  instances  to  discover  when  a  group  of 


The  Evolution  of  Plants 


321 


plants  appeared  and  how  the  group  became  more  widely  dis- 
tributed and  more  highly  diversified  in  form  and  structure. 
Some  of  these  ancient  plant 
groups  that  at  one  time  formed 
a  considerable  part  of  the  earth's 
vegetation  have  entirely  disap- 
peared;  others  are  now  repre- 
sented by  only  a  few  species. 

These  facts  all  lead  to  the 
conclusion  that  existing  plants 
have  been  derived  from  those 
of  the  past.  The  flowering 
plants  are  the  culmination  of 
a  long  series  of  constructive 
changes  in  plants,  which  en-  FlG  I03.  Fossii  imprint  of  a  leaf  of  a 

abled    them  to  live   in  a  greater    species  of  sassafras  in  rock  of  the  Cre- 
.  .  T         taceous  period.     The  Cretaceous  rocks 

variety    of    environments.     In-  were  forraed  much  later  than  the  Car. 

Creasingly  Complex  and  efficient    boniferous  rocks,  and  it  is  only  in  these 
£  .•        i«r  j    later  rocks  that  the   fossil   remains  of 

structures  for  vegetative  life  and  Angiosperms  are  found. 
reproduction    were    developed, 

and  these  structures  made  the  plants  better  able  to  endure 
unfavorable  conditions.  The  progressive  changes  in  plant 
life  have  occurred  during  a  geological  history  extending  through 
many  millions  of  years,  and  it  now  seems  impossible  to  account 
for  the  geological  record  except  on  the  basis  of  evolution. 
Certainly  no  one  has  suggested  any  other  plausible  way  to 
explain  the  long  series  of  gradually  changing  fossil  forms  that 
begins  with  simple  fernlike  plants  and  ends  in  plants  like  those 
found  on  the  earth  today. 

Geographic   distribution   and   evolution.     When   the   geo- 
graphic distribution  of  plant  families  is  studied,  it  becomes 


322 


Science  of  Plant  Life 


The  Evolution  of  Plants  323 

apparent  that  the  species  belonging  to  any  particular  group 
are  not  scattered  haphazard  over  the  earth.  Many  families 
bear  evidence  of  having  originated  on  some  particular  con- 
tinent or  part  of  a  continent,  and  of  then  having  spread 
from  the  center  of  origin.  Commonly,  in  the  center  of  origin 
there  is  or  was  the  greatest  variety  of  species,  and  away  from 
the  center  the  species  are  fewer  and  less  varied.  For  example, 
the  Cactus  family,  represented  by  more  than  a  thousand 
species,  is  native  in  North  and  South  America  only.  In 
North  America  the  family  is  best  developed  in  Mexico,  but 
it  has  spread  northward  and  eastward  into  the  United  States 
and  to  the  islands  of  the  West  Indies.  The  geographic  dis- 
tribution of  all  the  North  American  species  points  to  a  com- 
mon origin  in  the  Mexican  plateau.  The  Yucca  family  and 
the  Agave  (century  plant)  family  also  appear  to  have  orig- 
inated there  and  to  have  spread  in  a  similar  way  to  the  United 
States  and  the  West  Indies. 

Sometimes  the  record  is  incomplete  because  the  families 
are  very  ancient  and  most  of  the  species  have  become  extinct. 
In  other  families  the  record  is  so  complete  that  it  is  possible 
from  the  distribution  to  trace  the  origin  and  relationship  of 
many  of  the  species.  The  geography  of  plants,  therefore, 
furnishes  a  second  line  of  evidence  that  existing  plant  species 
have  been  derived  from  preexisting  forms. 

Similarity  of  structures  of  plants  belonging  to  diverse 
groups.  One  of  the  most  striking  proofs  of  evolution  is  found 
in  the  remarkable  similarity  of  the  cells,  tissues,  and  organs 
that  make  up  plants  belonging  to  very  different  groups.  The 
conductive  system,  for  example,  is  not  very  different  in  the 
ferns  and  in  the  Angiosperms.  The  bundles  are  arranged  dif- 
ferently in  these  groups,  and  their  arrangement  becomes  in- 


324  Science  of  Plant  Life 

creasingly  more  complex  from  the  ferns  to  the  flowering  plants ; 
but  the  cells  of  which  the  bundles  are  composed  are  very 
similar.  The  reproductive  structures  also  show  a  gradual 
modification  in  plants.  They  are  comparatively  simple  in 
the  ferns  and  very  complex  in  the  flowering  plants,  and  be- 
tween the  two  extremes  there  are  many  gradations.  These 
gradations  occur  in  such  an  order  that  they  point  to  the  grad- 
ual coming  into  existence  of  complex  reproductive  structures, 
culminating  in  those  of  insect-pollinated  flowers. 

Life  histories  of  plants.  Again,  when  we  compare  the  de- 
tailed life  histories  of  plants  belonging  to  the  different  groups, 
the  close  resemblances  among  them  point  to  their  origin  by 
gradual  evolution  from  common  ancestors.  As  one  passes 
from  the  algae  through  the  mosses  and  ferns  to  the  Gymno- 
sperms  and  Angiosperms,  the  life  histories  of  the  plants,  like 
their  structures,  become  more  and  more  complicated.  This 
increase  in  complexity  of  the  life  history  is  brought  about  by 
gradual  modification  and  by  the  addition  of  new  steps  and 
new  structures.  The  life  histories  of  related  groups  are 
similar  in  essentials  and  differ  only  in  details.  This  repetition 
of  the  stages  in  the  life  cycles  of  the  plants  of  different  groups 
can  be  explained  only  by  assuming  that  the  plants  with  the 
more  complex  life  histories  have  evolved  from  those  with  less 
complex  life  histories.  Increase  in  complexity  is  one  of  the 
general  tendencies  of  evolution.  The  order  in  which  we  should 
arrange  plants  on  the  basis  of  the  geological  records  is  the  same 
as  the  order  suggested  by  their  life  histories  and  structures. 

Intergrading  species.  All  who  have  attempted  to  classify 
plants  —  that  is,  to  determine  the  species  to  which  individual 
specimens  belong  —  have  been  impressed  by  the  intergrading 
of  related  species.  The  existence  of  individuals  intermediate 


The  Evolution  of  Plants  325 

between  species  long  ago  suggested  that  the  one  species  may 
have  arisen  from  the  other.  For  example,  the  common  asters, 
violets,  hawthorns,  evening  primroses,  and  willows  are  highly 
variable,  and  it  is  frequently  impossible  definitely  to  classify 
a  particular  specimen  and  to  say  that  it  belongs  to  this  or 
that  species.  If  such  intermediate  forms  were  rare,  they 
would  only  suggest  the  possibility  of  evolution ;  but  they  are 
numerous,  occurring  in  hundreds  of  families  throughout  the 
plant  kingdom.  These  intergrades  make  it  impossible  for 
us  to  think  of  the  plant  kingdom  as  being  composed  of  dis- 
tinct and  unrelated  species,  and  so  they  must  be  regarded  as 
evidences  of  evolution. 

Plant  breeding  and  evolution.  If  we  study  the  histories  of 
various  cultivated  plants,  —  for  example,  the  many  varieties 
of  cabbage,  tomatoes,  corn,  wheat,  apples,  peaches,  chrysan- 
themums, and  asters,  —  we  find  abundant  evidence  that 
they  have  been  derived  from  wild  plants.  The  plant  breeder 
has  simply  selected  and  preserved  desirable  forms  that  have 
arisen  by  mutation  and  through  hybridization. 

When  we  realize  the  extent  to  which  these  plants  have  been 
modified  from  the  types  of  their  wild  ancestors  in  the  com- 
paratively short  time  that  plants  have  been  cultivated,  it  is 
less  difficult  to  understand  the  great  changes  in  plant  life 
that  may  have  occurred  during  the  long  period  since  plants 
first  grew  on  the  earth. 

Plant  breeding  furnishes  us  with  experimental  evidence  that 
plants  may  change  in  various  ways,  so  that  not  all  the  plants 
of  succeeding  generations  are  like  their  ancestors.  Here  and 
there  an  individual  arises  that  is  different  from  its  parents, 
and  such  new  individuals  are  the  beginnings  of  new  races, 
varieties,  and  perhaps  species.  The  experience  of  plant 


326  Science  of  Plant  Life 

breeders  is  in  harmony  with  the  theory  of  evolution,  and 
plant  breeding  has  furnished  us  suggestions  as  to  the  methods 
by  which  evolution  has  taken  place. 

Evolution  proved;  methods  of  evolution  not  proved. 
Among  the  thousands  of  students  of  modern  and  ancient 
plants,  there  are  few,  if  any,  who  do  not  regard  evolution  itself 
as  a  fact.  The  evidence  points  so  clearly  toward  evolution, 
and  all  other  explanations  for  the  origin  of  the  plant  species 
now  on  the  earth  have  proved  so  inadequate,  that  botanists 
consider  evolution  as  proved.  At  the  same  time,  we  know 
comparatively  little  today  concerning  the  agencies  that  have 
brought  about  evolution.  Evolution  as  a  process  has  been 
studied  only  recently,  and  merely  a  beginning  has  been  made 
in  unraveling  the  tangled  skein  of  causes  and  methods  by 
which  modern  plants  have  come  to  be  what  they  are. 

Factors  of  evolution.  When  we  seek  to  learn  the  reasons 
for  the  changes  in  plant  life  during  geological  times,  we  find 
that  many  factors  were  probably  involved.  We  can  get  an 
idea  of  some  of  the  possible  factors  from  suggestions  that  have 
been  made  by  students  of  evolution,  but  we  must  keep  in 
mind  the  fact  that  there  is  today  little  real  evidence  concern- 
ing the  factors  of  evolution  and  that  the  suggestions  merely 
indicate  how  evolution  may  have  come  about.  It  seems 
probable,  however,  that  in  the  production  of  new  kinds  of 
plants  variation  and  selection  play  a  part.  Certainly  there 
may  be  among  the  offspring  of  a  plant  individuals  that  differ 
from  the  parent,  and  some  variations  are  known  to  be  inher- 
itable. In  order  to  explain  evolution,  it  is  necessary  to  assume 
that  new  characters  are  produced  in  plants  by  variation,  and 
that  the  changes  in  the  plants  are  transmitted  from  genera- 
tion to  generation. 


The  Evolution  of  Plants  327 

Natural  selection.  Most  plants  produce  offspring  by  the 
hundreds,  thousands,  or  even  millions,  and  there  is  room  for 
only  a  small  part  of  the  offspring  to  live.  It  is  said  that  those 
plants  survive  that  are  more  vigorous,  that  are  better  ad- 
justed to  their  environments,  or  that  happen  to  start  in  favor- 
able places;  the  weak  and  the  unfortunate  perish.  Certain 
variations,  or  mutations,  may  fit  plants  the  better  to  survive, 
and  the  persistence  of  the  forms  showing  these  changes  may 
lead  to  the  formation  of  new  varieties  and  species.  The 
wholesale  destruction  of  individual  plants  in  nature,  with  the 
survival  of  a  few,  is  called  Natural  Selection,  and  it  has  been 
thought  to  resemble  in  some  respects  the  selection  made  by 
the  plant  breeder.  It  is  unquestionably  true  that  most  of 
the  plants  that  start  life  in  nature  die  before  reaching  ma- 
turity ;  but  there  are  great  differences  of  opinion  as  to 
whether  or  not  the  plants  that  do  survive  can  through  re- 
peated selections  in  nature  develop  into  new  species.  Man 
can  pick  out  new  forms  that  originate  in  the  plants  that  he 
cultivates  and  by  breeding  from  them  secure  new  varieties, 
but  it  is  believed  by  some  that  in  nature  the  variations 
would  be  lost  by  interbreeding  with  the  parent  forms. 

Changes  in  environment.  Another  factor  in  Natural 
Selection  is  the  fact  that  climates  and  environments  are  ever 
changing.  We  know,  for  example,  that  for  a  long  period 
of  time  tropical  plants  grew  in  polar  regions,  for  the  fossils 
of  tropical  plants  are  now  found  there  in  the  rocks.  At  a 
later  period  the  climate  of  the  earth  was  much  colder  than 
it  is  now,  and  all  of  northern  North  America  was  covered 
with  ice  that  reached  as  far  south  as  the  Ohio  and  Missouri 
rivers.  As  climates  change,  plants  either  become  adjusted 
to  the  changes  or  perish.  These  changes  in  climate  afford 


328  Science  of  Plant  Life 

opportunities  for  new  sets  of  plants  to  take  possession  of  the 
land.  They  are  perhaps  also  the  cause  of  mutations  in  plants 
that  give  rise  to  new  forms  which  replace  the  species  from 
which  they  were  derived. 

The  land  areas  have  changed  many  times  in  the  earth's 
history.  Sometimes  large  parts  of  the  continents  have  been 
under  water ;  at  other  times  the  continents  have  been  more 
elevated  than  at  present.  These  changes  in  the  land  areas 
have  resulted  in  the  extinction  of  many  species  of  plants. 
Forms  derived  from  them  may  have  survived  on  the  land  not 
submerged.  So  the  great  changes  in  climate  and  in  land 
surfaces,  and  the  less  pronounced  changes  in  plant  habitats, 
act  as  selective  agencies,  determining  the  kinds  of  plants  that 
survive. 

Isolation.  If  a  continuous  land  area,  having  a  number  of 
elevations,  were  submerged  so  that  the  elevations  became 
islands,  a  plant  that  had  been  originally  distributed  over  the 
whole  area  might  persist  on  a  number  of  the  islands.  The 
plants  on  the  several  islands  would  then  be  isolated  from  one 
another.  As  time  went  on  these  isolated  individuals  might 
give  rise  to  new  forms  that  were  quite  distinct  in  their  char- 
acteristics. Different  new  forms  might  arise  on  different 
islands,  and  since  the  selective  agencies  on  the  various  islands 
would  not  be  the  same,  the  varieties  that  persisted  on  one 
island  might  come  to  be  quite  distinct  from  those  on  other 
islands.  The  production  of  the  new  forms  depends  on  vari- 
'ation,  but  isolation  tends  to  preserve  new  forms  by  preventing 
their  spreading  and  interbreeding. 

The  deciduous  forest  of  eastern  North  America  was  for- 
merly continuous  with  the  forests  of  China  through  Alaska. 
Since  then  the  connecting  land  has  been  depressed,  and  the 


The  Evolution  of  Plants  329 

two  continents,  Asia  and  North  America,  have  become 
separated  and  their  climates  have  changed.  This  separa- 
tion resulted  in  the  isolation  of  the  deciduous  forest  plants 
of  eastern  Asia  from  those  of  eastern  America.  Many  of 
the  same  species  are  still  found  in  both  areas.  But  in  the 
two  widely  separated  regions  there  are  many  other  quite 
distinct  species  that  have  arisen  by  evolution  from  a  com- 
mon ancestor.  The  American  sycamore,  or  example,  is 
distinct  from  the  Chinese  sycamore ;  but  the  resemblance 
between  the  two  is  sufficient  to  suggest  that  they  have  a  com- 
mon ancestry. 

Variations;  the  elimination  of  many  individuals  through 
lack  of  room  in  which  to  grow ;  changes  in  climates  and  in  land 
areas ;  and  isolation  are  believed  to  be  important  factors  in 
the  evolution  of  plants. 


INDEX 


Abscission,  67. 

Absorption,  170;  and  rise  of  sap,  135,  176; 

and  transpiration,  176. 
Accumulation,  of  food,  74,  75,  77,  105,  125, 

159,  !79;  °f  water,  58,  106,  158. 
Aconite,  195. 

Acre,  food  products  from  an,  34. 
Advantages,   of  deciduous  habit,   63 ;    of 

horizontal    stems,     104;     of    spreading 

roots,  169;  of  upright  stems,  102. 
Aerial  roots,  189. 
Agar,  246. 

Agave,  58,  90,  91,  97- 
Agriculture,  4,  99 ;  and  food  accumulation, 

78. 

Alcohol,  261. 
Alder,  198,  201. 
Alfalfa,  194. 
Algae,   234 ;    blue-green,   245 ;    brown  and 

red,    246;     living    conditions    of,    239; 

reproduction  among,  243;    structure  of, 

234- 

Alligator  pear,  212. 
Amanita,  267. 
Amaryllis,  161. 
Angiosperms,  211,  303;   divisions  of,  304; 

seeds  of,  211. 
Animals,  149,  231. 

Annual  growth,  129;  rings,  123,  129. 
Annuals,  94. 
Anther,  200. 
Anthocyan,  65. 
Antitoxins,  253. 
Apple,  76,  79;  May,  98. 
Artichoke,  214;  Jerusalem,  97,  161,  163. 
Arts,  plant-producing,  99. 
Asparagus,  163. 
Assimilation,  82,  85. 
Autophytes,  249. 
Autumn  coloration,  65. 
Avens  leaf,  15. 
Avocado,  212. 
Axil,  106. 

Bacteria,  2,  250;    and  disease,  253;    and 

legumes,  194,  258;   and  sanitation,  252; 

and  soils,  257;    and  soil  nitrogen,  258; 

in  the  dairy,  256. 
Bamboo,  164;    leaf,  14;    stem,  124,  130, 

303- 

Banana,  79,  97,  219. 
Bark,  129,  170. 
Bast,  119;   fibers,  123. 
Bean,  43,  207,  223;  soy,  33,  78. 
Beech,  194. 
Beets,  96,  194. 
Begonia  leaf,  14. 


Benedict's  solution,  73. 
Bermuda  grass,  159. 
Biennials,  94,  95,  96. 
Black  locust,  198. 

Blade,  13,  14;  position  and  light,  41. 
Bloom,  54. 
Blue-green  algae,  245. 
Boston  fern,  226;  ivy,  182,  189. 
Botany,  3,  4;  subdivisions,  151. 
Bread  making,  261. 
Breeding,  plant,  220. 
Bromelias,  189,  191. 
Brown  color  of  leaves,  66. 
Bryophyllum,  218. 
Buckeye  leaf,  15;   twig,  107. 
Buds,  1 06 ;  and  plant  form,  109 ;  kinds  of, 
109,  160;    opening  of,  108;    scars,  107, 

113,  114- 
Budding,  132. 
Bulbs,  97,  160,  219. 
Bulrushes,  157. 

Bundle,  dicot,  121;  monocot,  126. 
Bundles,  leaf,  20;    root,  169;    stem,  120, 

121,  125,  126,  128. 
Burdock,  102. 

Cabbage,  96,  225. 

Cactus,  56,  158,  190;   spineless,  6. 

Caladium,  160,  219. 

Calorie,  34. 

Calyx,  200. 

Cambium,  119,  124;  and  grafting,  132. 

Camphor,  163. 

Canna,  219. 

Capsule,  212. 

Carbohydrates,  27,  135. 

Carboniferous  plants,  283. 

Carotin,  63. 

Carrot,  96,  186,  198;    family,  315;    wild, 

316. 

Castor  bean,  207. 
Catkin,  198. 
Cat-tail,  157,  198. 
Cauliflower,  214,  225. 
Celandine  leaf,  15. 
Celeriac,  194. 
Cells,  15,  1 6,  21 ;   guard,  18;    tissues  and 

organs,  21,  178. 
Cell  sap,  1 6. 
Cellulose,  16,  86,  100. 
Cell  walls,  1 6,  276;  making  of,  86. 
Century  plant,  58,  76,  96,  97. 
Cereus,  58. 
Chlorophyll,  22,  63. 
Chloroplasts,  22. 
Cion,  132. 
Cladonia,  269. 


331 


332 


Index 


Clavaria,  267. 

Cleft  grafting,  132. 

Climbers,  155. 

Closed  bundle,  126. 

Clover  leaf,  16,  44;  root,  194. 

Club  mosses,  286. 

Coconut,  214,  215,  306. 

Color  of  leaves,  43,  45. 

Commercial    products,    from   leaves,    91 ; 

from  roots,  194;  from  stems,  161. 
Compass  plants,  40. 
Composite  family,  315. 
Concord  grape,  225. 

Conduction,  food,  105,  121;  water,  121. 
Conductive  system,   121;    significance  of, 

290. 
Conductive  tissues,  leaves,  20;  roots,  169; 

stems,  105,  121. 
Conifer  forests,  298. 
Conifers,  118,  294;    leaves  of,  296;    seeds 

of,  296;  stem  structure  in,  128. 
Copra,  214. 
Cork,  163. 
Corm,  160,  219. 
Corn,  8,  50,  78,  79,  142,  164,  167,  179,  199, 

202,   205,   207,   209,   215,   221,   222. 

Corolla,  200. 

Cortex,  119,  1 20. 

Cotton  fiber,  215. 

Cotyledon,  207. 

Cowpeas,  194. 

Craterellus,  267. 

Crop  yields,  34,  230 ;  and  water  supply,  51. 

Cucumber,  wild,  155. 

Cuticle,  1 8,  54. 

Cutin,  17. 

Cuttings,  167. 

Cypress,  296. 

Cytoplasm,  15,  175. 

Dahlias,  97,  317. 

Dandelion,  103,  166,  229;   leaf,  15;   root, 

167. 

,   Dasheen,  160,  163. 
Date  palm,  no. 

Deciduous  forest,  305,  312;  habit,  63. 
Deliquescent  stems,  in. 
Desert  plants,  58,  59,  154,  157. 
Development  of  plant  kingdom,  300. 
Diastase,  75. 
Diatoms,  244. 

Dicots,  118,  209;  stem  structure,  120. 
Dicotyledons,  families  of,  311. 
Diffusion,  171. 
Digestion,  74. 
Diseases,  bacterial,  253. 
Dodder,  250. 


Dogwood,  39. 
Dulse,  246. 

Duration,  of  leaves,  69 ;  of  stems,  94 ;  of 
roots,  185. 

Ecological  factors,  140,  151;  types  of 
leaves,  42,  43,  44,  56,  57  ;  types  of  roots, 
1 86  ;  types  of  stems,  154. 

Ecology,  151. 

Economic  importance,  of  algae,  245 ;  of 
bacteria,  251;  of  flowers,  214;  of  fruits, 
214;  of  pond  scums,  245;  of  seeds, 
214. 

Eel  grass,  57,  91. 

Egg  cells,  205. 

Elements  of  soil,  141. 

Elodea,  28,  29,  57. 

Embryo,  206. 

Endosperm,  207. 

Energy,  82. 

Environment,  8,  139;  changes  in,  327; 
complexity  of,  150;  factors  of,  139; 
responses  to,  130,  131,  275. 

Enzymes,  74. 

Epidermis,  15,  17,  50,  276;  importance  of, 
18;  stem,  120;  root,  169. 

Epiphytes,  1 88,  189. 

Equisetums,  285. 

Eucalyptus,  99. 

Euphorbia,  58. 

Evening  primrose,  186. 

Evergreen  habit,  69,  71. 

Evergreens,  64,  69. 

Evolution,  300,  318;  and  geographic  dis- 
tribution, 321 ;  and  intergrading  species, 
324;  and  life  histories,  324;  and  plant 
breeding,  325 ;  and  plant  structures, 
323  ;  factors  of,  326. 

Excurrent  stems,  in. 

Fabrics  from  plants,  i. 

Factors,  of  evolution,  326;  of  the  environ- 
ment, 139. 

Fats,  production  of,  31. 

Fermentation,  261. 

Ferns,  59,  98,  298;  leaves  of,  15,  88,  185, 
283;  life  history  of,  288;  number  of, 
303 ;  walking,  283. 

Fertilization,  205;  in  conifers,  296;  in 
ferns,  289;  in  mosses,  280. 

Fiber,  i,  91,  215,  228;  Manila,  91. 

Filament,  200. 

Flax,  163. 

Floral  envelope,  200. 

Floriculture,  100. 

Flower,  198,  200;  clusters,  198;  dicot, 
209;  monocot,  209. 


Index 


333 


Flowers,  variety  of,  200;  economic  im- 
portance of,  214. 

Fog,  148. 

Fomes,  245. 

Food,  25 ;  per  acre,  33. 

Food  accumulation,  and  agriculture,  78; 
in  roots,  77,  179;  in  leaves,  75;  in 
stems,  105,  159,  161. 

Food-conducting  tissue,  20. 

Food  conduction,  21,  178. 

Foods,  produced  per  acre,  33 ;  utilization 
of,  82. 

Forests  of  North  America,  299,  312. 

Fossils,  319. 

Fructose,  28. 

Fruit,  212;  economic  importance  of,  214. 

Fuel,  2. 

Fungi,  260;  and  roots,  194. 

Gametes,  241. 

Geaster,  267. 

Gemmae,  273. 

Geographic  distribution,  321. 

Geotropism,  131,  182. 

Ginseng,  195. 

Glucose,  27,  28. 

Grafting,  132. 

Grape,  225. 

Grass  family,  308. 

Gravity,  140,  149,  182. 

Ground  pine,  287. 

Growing  point,  129,  180. 

Growing  region,  88,  1 29,  180 ;  of  leaves,  87  ; 
of  roots,  180;  of  stems,  129. 

Growth,  82,  85;  conditions  for,  87;  de- 
termination of  annual,  114;  of  roots, 
179;  of  stems,  128;  pressure,  182,  184; 
stages  of,  1 80. 

Guard  cells,  18. 

Guayule,  163. 

Guinea  grass,  42. 

Gymnosperms,  211,  294;  number  of,  303; 
seeds  of,  211 ;  wood  of,  294. 

Habitat,  56. 

Heartwood,  130. 

Heath  family,  315. 

Hemp,  163. 

Herbs,  94 ;  shrubs  and  trees,  98. 

Hippuris,  56. 

Horseradish,  167,  219. 

Horsetails,  286. 

Horticulture,  4,  100. 

Host,  250. 

Humus,  143,  257. 

Hyacinth,  97,  160. 

Hybridization,  226,  255. 


Hybrids,  220,  226. 

Hydnum,  267. 

Hydrophytes,  56,  57,  60,  156,  179,  273. 

Hydrotropism,  131,  183. 

Hypocotyl,  207. 

Imbibition,  172. 

Indian  cucumber  root,  38. 

Indian  pipe,  249. 

Insect  pollination,  104,  202. 

Internodes,  '106. 

lodin,  246. 

Ipecac,  195. 

Iris,  41. 

Irish  moss,  246. 

Irrigation,  141,  144,  145. 

Isolation,  328. 

Jack-in-the-pulpit,  160. 
Jerusalem  artichoke,  97,  161,  163. 
Johnson  grass,  159. 
Jute,  163. 

Kelps,  246. 
Kohl-rabi,  163. 

Landscape  architecture,  100. 

Lantana  leaf,  14. 

Laurel,  316. 

Leaf,  arrangement,  37 ;  coloration,  63 ; 
fall,  67  ;  parts  of,  13  ;  pigments,  63,  65  ; 
position  and  light,  37,  46;  scars,  113; 
structure,  13,  42,  54,  56;  tissues,  14. 

Leaves,  13  ;  commercial  products  from,  91 ; 
effects  of  light  on,  42  ;  floating,  57  ;  hori- 
zontal, 43  ;  motile,  43  ;  of  conifers,  296 ; 
of  shade  plants,  44 ;  relation  to  light,  37, 
46 ;  submerged,  45,  56 ;  vertical,  42 ; 
water  relations  of,  48 ;  white,  66. 

Legume  family,  313. 

Legumes  and  bacteria,  194,  258. 

Lenticels,  106,  115. 

Lichens,  59,  269. 

Lifting  of  water,  134. 

Light,  148. 

Lignification,  123. 

Lignin,  122. 

Lily  family,  310. 

Lipase,  75. 

Liverworts,  272. 

Longevity  of  plants,  94. 

Lumbering,  161. 

Lycopodium,  287. 

Magnolia,  37. 
Mango,  211. 


334 


Index 


Mangrove,  seedlings,  208;  swamp,  209. 
Maple,   30,   38,    201;    flow  of    sap,    135; 

sugar,  30,  163. 
Marchantia,  272,  275. 
May  apple,  98,  159. 
Mechanical  tissue,  21. 
Membranes,  173. 
Mertensia,  54. 
Mesophyll,  15,  19. 
Mesophytes,  60. 
Microspora,  236,  238. 
Midrib,  13. 
Milkweed,  198. 
Mimosa,  44,  314. 
Mint  family,  314. 
Mistletoe,  249. 
Mm'um,  280. 
Molds,  262. 
Monocots,    1 1 8,    209;     families    of,    307; 

stem' structure,  125. 
Morphology,  151. 

Mosses,  23,  59,  277 ;   life  history  of,  279. 
Moth  mullein,  95. 
Motile  leaves,  43. 

Multiplication,  vegetative,  217,  243,  279. 
Mushrooms,  249,  267,  268. 
Mustard  family,  312. 
Mutants,  225. 
Mutation,  224,  327;  bud,  226. 

Naiad,  57. 

Nasturtium  leaf,  14. 

Natural  selection,  327. 

Nectar,  202.  ^ 

Nectaries,  202. 

Net- veined  leaves,  88,  118. 

Nightshade,  315. 

Nitrifying  bacteria,  258. 

Nitrogen,  32,  141 ;  fixation,  258. 

Nodes,  106. 

Nucleus,  15. 

Nutritive  phase  of  plants,  197. 

Oak,  161,  198. 

Oats,  147. 

Oedogonium,  236,  241. 

Oils,  31,  215. 

Onion,  79,  160,  198. 

Oospore,  242. 

Open  bundle,  127. 

Orange  leaf,  14. 

Orchid  family,  311. 

Orchids,  190,  204,  311. 

Organs,  21. 

Osier  willow,  163. 

Osmosis,  173;  in  roots,  175. 

Ovulary,  200. 


Ovule,  200. 
Oxalis  leaf,  15,  44. 
Oxidation,  82. 

Palisade  cell,  17;  layers,  19. 

Palm,  coconut,  214,  306;  date,  no; 
family,  307;  hat,  91. 

Parallel- veined  leaves,  88,  118;  growing 
region  of,  87. 

Parasites,  249. 

Parenchyma,  120. 

Parmelia,  269. 

Parsnip,  79,  96. 

Pasteurization,  256. 

Pea  leaf,  16. 

Peach  leaf,  14. 

Peanut,  78,  79. 

Pedicel,  200. 

Peduncle,  199. 

Peppermint,  314. 

Pepsin,  75. 

Perennials,  96. 

Permeability,  173. 

Persimmon,  213. 

Petal,  200. 

Petiole,  14. 

Phloem  =  food  conducting  and  bast  tis- 
sues, 119. 

Photosynthesis,  26;   summary  of,  31. 

Phototropism,  131,  183. 

Physiology,  151. 

Pigments,  green,  63  ;    red,  65  ;   yellow,  63. 

Pine,  202;  Japanese  dwarf ,  99 ;  trees,  in. 

Pineapple,  210. 

Pistil,  200. 

Pith,  120,  125;  rays,  120. 

Plant,  breeding,  220;  characteristics,  99; 
fibers,  i,  91,  215,  228;  kingdom,  de- 
velopment of,  300;  producing  arts,  99. 

P'ants,  desert,  58,  157;  importance  of,  i; 
living  things,  7,  8;  mutual  dependence 
of  oarts,  8,  9;  of  the  past,  318;  relation 
tc  environment,  8,  140;  source  of  all 
food,  i  2.5. 

Plastids,  22. 

Plumule,  208. 

Pod,  212. 

Pollen,  200,  205 ;   tube,  204. 

Pollination,  201 ;  cross,  203 ;  in  conifers, 
202  ;  self,  203. 

Pond  scums,  237,  245  ;  living  conditions  of, 
239- 

Pondweed,  57. 

Poplar  leaf,  14. 

Potassium,  source  of,  247. 

Potato,  10,  76,  79,  87;  sweet,  97,  194; 
tuber  cell,  17;  family,  315. 


Index 


335 


Preservation  of  foods,  257. 

Pressure,  of  growth,   184;    osmotic,   174; 

root,  177. 

Prickly  lettuce,  40 ;  pear,  58. 
Proteins,  making  of,  32;  use  of,  32,  33. 
Prothallus,  288. 
Protococcus,  235. 
Protonema,  279. 
Protoplasm,  151. 
Puffballs,  267. 
Pulvinus,  45. 

Raceme,  198. 

Rainfall,  distribution  of,  51,  144;  effect 
on  yield  of  corn,  48,  51. 

Raspberry,  213,  217. 

Rattan,  125,  163. 

Receptacle,  200. 

Red-bud  tree,  44. 

Red  pigment,  65. 

Reproduction,  10,  197,  206;  and  agri- 
culture, 217;  asexual,  243;  by  seeds, 
207,  300;  sexual,  205,  243. 

Reproductive  phase  of  plant  life,  197. 

Reservoirs,  cleaning,  246. 

Respiration,  82;  and  shipping,  84;  con- 
trasted with  photosynthesis,  83;  in 
roots,  82,  179;  rate  of,  84. 

Response,  131 ;  to  aerial  environment, 
2755  to  gravity,  130,  149,  182;  to  light, 
131,  149,  183 ;  to  water,  131,  157,  183. 

Rhizoids,  273,  276 ;  moss,  278. 

Rhododendrons,  194. 

Rhubarb,  195,  198. 

Rice,  77,  145,  307,  308. 

Root,  contraction,  185;  hairs,  168;  pres- 
sure, 177;  processes,  166. 

Roots,  and  bacteria,  194,  258;  and  fungi, 
194;  and  transplanting,  190;  classifica- 
tion of,  1 66;  commercial  uses  of,  194; 
distribution  of,  183 ;  of  climbers,  182, 
187;  of  conifers,  296;  of  ferns,  285;  of 
hydrophytes,  187  ;  of  mesophytes,  187  ; 
response  to  gravity,  182;  response  to 
light,  183  ;  tissues  of,  169. 

Rootstock,  159,  219;   calamus,  122. 

Rose  family,  312. 

Rosin,  161,  162. 

Rubber,  163,  215. 

Running  cypress,  287. 

Rushes,  157. 

Russula,  267. 

Rusts,  263. 

Salsify,  194. 
Sand-reed  grass,  159. 
Sanitation,  252. 


Saprophytes,  249. 

Sapwood,  130. 

Sassafras,  195. 

Scale  leaves,  160,  161. 

Science,  defined,  3. 

Seaweeds,  246. 

Sedges,  38,  157. 

Seeds,  198,  205,  207 ;  economic  impor- 
tance of,  214;  of  conifers,  198,  211. 

Selaginella,  59. 

Selection,  220,  228;  natural,  327. 

Self-pruning,  69. 

Sensitive  plant,  44. 

Sepal,  200. 

Sequoia,  98,  154,  229. 

Shasta  daisy,  222. 

Shrubs,  94. 

Sieve  tubes,  122. 

Silviculture,  100. 

Simple  plants,  summary  of,  270. 

Sirup,  163. 

Sisal,  90,  91. 

Smilax,  107. 

Smuts,  266. 

Snapdragon,  198. 

Soil,  141 ;  bacteria,  257 ;  cultivafton  of, 
143 ;  elements,  141 ;  erosion,  186,  187 ; 
temperature,  146. 

Solomon's  seal,  98,  159,  185. 

Sorghum,  163. 

Soy  bean,  33,  78,  194. 

Sperms,  205. 

Spike,  198. 

Spirogyra,  236. 

Spores,  asexual,  241 ;  resting,  238;  sexual, 
241 ;  swimming,  238. 

Sports,  225;  bud,  226. 

Spruce,  114. 

Stamen,  200. 

Starch,  28,  74,  77. 

Stem,  processes,  118;    structure,  118,  119, 

!54- 

Stems,  advantages  of  horizontal,  104;  as 
photosynthetic  organs,  106 ;  commercial 
products  from,  161 ;  excurrent  and  deli- 
quescent, 1 1 1 ;  external  features  of,  102  ; 
growth  of,  128 ;  in  relation  to  light,  131 ; 
of  climbing  plants,  155  ;  of  hydrophytes, 
156;  of  xerophytes,  157 ;  underground, 
97,  130;  upright,  102. 

Stigma,  200. 

Stipules,  14. 

Stock,  132. 

Stomata,  19,  115. 

Storage,  food,  74,  77,  105,  125,  159,  160, 
161 ;  water,  58,  106,  158,  189. 

Strawberry,  10. 


336 


Index 


Structure,  conifer  stems,  128 ;  dicot  bundle, 

121 ;  dicot  stems,  120;  monocot  bundle, 

126;   monocot  stem,  125. 
Style,  200. 
Subsidiary  cells,  19. 
Sugar,  28,  202;    beet,  77,  79,  195;    cane, 

77,  163,  219. 

Sugar  cane,  77,  78,  141,  163,  231. 
Sunflower,  10,  50,  97,  102,  198,  203,  222, 

228,  317. 
Synthesis,    of  carbohydrates,   26,   27,  31 ; 

of  fats,  31 ;  of  proteins.  32. 

Tamarack,  296. 

Tank  epiphytes,  189. 

Tannic  acid,  161. 

Temperature,  146. 

Tendrils,  107,  155. 

Test,  for  oil,   73 ;    for  proteins,   73 ;    for 

starch,  26,  73  ;  for  sugar,  73. 
Thallus,  272. 
Thistle,  317. 
Timothy,  220. 
Tissues,   21 ;    epidermal,   15,    16,   50,    119, 

169;     food-conducting,     15,    121,     169; 

mechanical,  15,  122;   storage,  125. 
Toadstools,  268. 
Tobacco,  89,  203,  224. 
Tomato,  225. 
Toxins,  253. 
Tracheae,  121,  123. 
Transfer  of  food,  76. 
Transpiration,  49,   149 ;    and  absorption, 

176;     and   lifting   of   water,    134;     and 

water  balance,  52  ;   rate  of,  50. 
Transplanting,  192;    and  roots,  190;    and 

water  balance,  55. 
Tree-of-heaven,  107. 
Trees,  94;   size  of,  99,  154. 
Tropisms,  131. 
Tubers,  97,  161. 
Tulip,  1 60,  200;  tree  leaf,  13. 
Turf, _  1 59. 
Turnips,  96,  194. 
Turpentine,  161. 
Typhoid  fever,  253. 

Ulothrix,  236,  240. 
Umbel,  198. 


Underground  stems,  75,  97,  130,  159,  160, 

161. 
Utilization  of  foods,  83. 

Vacuoles,  16,  49. 

Variations,  220,  222,  327;   kinds  of,  223. 

Vaucheria,  236. 

Vegetative  multiplication,  217,  243,  279; 
propagation,  218. 

Veins,  13,  20,  88;   principal,  13. 

Vertical  leaves,  41,  42. 

Vessels,  food-conducting,  121 ;  water-con- 
ducting, 121,  123. 

Vinca,  14,  18,  21. 

Vinegar  making,  261. 

Walnut,  79. 

Wandering  Jew,  19. 

Water,  balance,  52,  54;  balance  and  plant 
habitats,  56  ;  balance  and  transplanting, 
55  ;  balance  illustrated,  53  ;  conducting 
tissue,  1 8,  20,  105,  119,  121,  169;  im- 
portance of,  48,  51,  52;  plants,  56,  57, 
60,  156,  244,  273;  storage,  58,  106,  158, 
189;  supply  and  crop  yields,  51;  why 
necessary,  48. 

Water  lily,  57;  yellow,  159. 

Weeds,  229;   and  crop  production,  230. 

Wheat,  5,  79,  96,  144;  rust,  263. 

White  clover,  44. 

White  leaves,  66. 

White  pine  blister  rust,  265. 

Willow,  16,  56,  163,  198. 

Wilt  disease,  254. 

Wind,  148,  149;   pollination,  103. 

Wintergreen  oil,  163. 

Wood,  123;  fibers,  124;  gymnosperm, 
294;  pulp,  161. 

Xanthophyll,  63. 
Xerophyte,  58,  59,  60,  157. 
Xylem  =  water    conducting    and     wood 
tissues,  119.  • 

Yam,  194. 
Yeast,  261. 
Yucca,  58,  204,  213,  322. 

Zebrina,  19. 


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It  contains  instructions  on  Capitalization,  Punctuation,  Italics,  Abbrevia- 
tions, Syllabication,  Compounds,  Spelling,  Construction  and  Criticism, 
Mechanical  Aids  and  Processes,  Letter- Writing,  Copy  for  the  Printer, 
Appendixes,  Bibliography.  Each  chapter  includes  material  valuable  for 
general  reference  or  for  some  special  need  as  well  as  the  regular  textbook 
material  intended  for  class  use. 

Cloth,    xii  +  266  pages 
INTERESTING  AS  A   NOVEL 

INTRODUCTION    TO    THE     STUDY 
OF   ENGLISH    LITERATURE 

By  VIDA  D.  SCUDDER,  A.M. 

Professor  of  English  Literature  at  Wellesley  College 

For  the  high  school  and  the  younger  classes  in  college.  It  gives  a  clear  and 
C9rrect  idea  of  each  great  period  of  English  Literature  and  guides  in  the 
direct  and  copious  reading  of  texts.  Each  part  begins  with  a  picture  of  the 
period  treated;  the  significance  of  our  origins  and  the  imaginative  achieve- 
ment of  the  great  medieval  centuries  are  clearly  shown  and  the  student  is 
enabled  to  trace  the  many  strands,  racial,  physical,  ethical,  and  spiritual, 
of  which  the  glorious  factric  of  English  Literature  is  woven. 
Special  prominence  is  given  to  the  greatest  and  most  significant  figures  in 
our  literature.  In  addition  to  the  bibliographical  references  at  the  end  of 
each  chapter  there  are  practical  suggestions  for  discussions  by  the  students 
and  for  talks  by  the  teacher.  A  full  outline  of  authors  with  their  works  and 
contemporary  events  in  tables  arranged  for  easy  reference  is  also  given. 
The  book  is  thoroughly  human,  interesting,  attractive,  and  inspiring,  and 
has  a  literary  charm  not  found  in  the  ordinary  textbook.  There  is  not  a 
dry  page  in  it  and  there  is  no  book  that  presents  the  story  of  English  Litera- 
ture in  a  manner  more  likely  to  awaken  the  desire  for  further  acquaintance 
with  the  books  that  are  so  alluringly  described. 

Cloth.    542  pages. 

WORLD    BOOK    COMPANY 

YONKERS-ON-HUDSON,   NEW  YORK 
2126    PRAIRIE   AVENUE,    CHICAGO 

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NEW-WORLD  SCIENCE  SERIES 
Edited  by  John  W.  Ritchie 

EXERCISE  AND  REVIEW 
BOOK  IN  BIOLOGY 

By  J.  G.  BLAISDELL 

A    Combined  Laboratory   Guide,   Notebook 
and-  Review  Book  for  Students'  Use 

TN  submitting  the  manuscript  of  this  manual  to  the 
A  publisher,  the  author  wrote: 

"As  a  teacher  of  biology,  I  have  for  several  years  felt  the 
need  of  a  laboratory  guide  and  notebook  that  would  lighten  the 
labor  and  economize  the  time  of  both  teacher  and  pupil  in  con- 
nection with  their  laboratory  work.  In  this  age  of  cheap  printing 
it  seems  a  needless  waste  of  time  and  strength  to  compel  teachers 
to  prepare  laboratory  outlines  and  to  mimeograph  or  copy  them 
on  the  blackboard,  and  it  ought  to  be  possible  to  permit  the 
pupil,  when  he  enters  the  laboratory,  to  begin  work  at  once  with 
his  laboratory  directions,  questions,  needed  outline  drawings,  and 
space  for  his  notes  and  sketches  all  on  one  neatly  printed  note- 
book page.  I  have  found  the  loose-leaf,  blank-page,  ring-cover 
notebook,  and  separate  laboratory  directions  method  both  waste- 
ful of  time  and  unsatisfactory  in  other  respects,  and  have  used 
careful  thought  and  my  experience  as  a  teacher  in  planning  a 
tetter  way." 

Mr.  Blaisdell  has  solved  the  problem  with  this  book.  His  manual 
bears  '.he  same  relation  to  laboratory  study  as  a  well-ordered 
text  bears  to  the  recitation,  and  cannot  fail  to  conserve  the  time 
of  both  teacher  and  pupil. 

It  contains  100  laboratory  exercises,  with  space  for  more,  cover- 
ing a  year's  work  in  general  biology,  planned  to  meet  the  re- 
quirements of  the  syllabus  issued  by  the  Regents  of  the  Uni- 
versity of  the  State  of  New  York,  and  to  accompany  any  high- 
school  text  in  general  biology  in  common  use. 

Review  exercises  are  provided,  and  a  series  of  Regents'  ques- 
tions are  so  arranged  as  to  give  a  review  of  the  year's  work. 

viii  +  152  pages      Price  $1.20 

WORLD  BOOK  COMPANY 

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